Organic Compound, Light-Emitting Element, Light-Emitting Device, Display Device, Electronic Device, and Lighting Device

ABSTRACT

A novel organic compound that forms an exciplex emitting light with high efficiency is provided. An organic compound with a triarylamine skeleton in which the three aryl groups of the triarylamine skeleton are a p-biphenyl group, a fluoren-2-yl group, and a phenyl group to which a dibenzofuranyl group or a dibenzothiophenyl group is bonded. By the use of the organic compound and an organic compound with an electron-transport property, an exciplex that emits light with extremely high efficiency can be formed.

TECHNICAL FIELD

The present invention relates to a light-emitting element, a displaydevice, a light-emitting device, an electronic device, and a lightingdevice each of which includes an organic compound as a light-emittingsubstance.

BACKGROUND ART

Advances are being made in application of a current excitation typelight-emitting element in which an organic compound is used as alight-emitting substance, i.e., an organic EL element, to light sources,lighting, displays, and the like.

As is known, in an organic EL element, the generation ratio of excitonsin a singlet excited state to excitons in a triplet excited state is1:3. Thus, the limit value of internal quantum efficiency offluorescence, which is emitted by conversion of a singlet excited stateinto light emission, is 25%, while phosphorescence, which is emitted byconversion of a triplet excited state into light emission, can have aninternal quantum efficiency of 100% when energy transfer via intersystemcrossing from a singlet excited state is taken into account. In view ofthe above, an organic EL element (also referred to as a phosphorescentlight-emitting element) in which a phosphorescent material is used as alight-emitting substance is selected in many cases so that light isemitted efficiently.

To cause conversion of a triplet excited state into light emission,delayed fluorescence can also be utilized. In this case, notphosphorescence but fluorescence is obtained because reverse intersystemcrossing from a triplet excited state to a singlet excited state isutilized and the light emission occurs from a singlet excited state.This is readily caused when an energy difference between a singletexcited state and a triplet excited state is small. Emission efficiencyexceeding the theoretical limit of emission efficiency of fluorescencehas been actually reported.

It has also been reported that an exciplex (excited complex) formed bytwo kinds of substances was utilized to achieve a state where an energydifference between a singlet excited state and a triplet excited stateis small, whereby a high-efficiency light-emitting element was provided.

REFERENCE Non-Patent Document

-   [Non-Patent Document 1] K. Goushi et al., Applied Physics Letters,    101, pp. 023306/1-023306/4 (2012).

DISCLOSURE OF INVENTION

However, in such a light-emitting element utilizing an exciplex, use ofcertain substances often prevents efficient light emission. Actually, inthe history of development of organic EL elements, an exciplex has beenconsidered to decrease efficiency and organic EL elements have beengenerally designed such that an exciplex is not formed.

Against this backdrop, structures for forming exciplexes thatefficiently emit light have hardly been determined.

In view of the above, an object of one embodiment of the presentinvention is to provide a novel organic compound which forms an exciplexthat efficiently emits light. Another object of one embodiment of thepresent invention is to provide a light-emitting element which has highemission efficiency. A further object of one embodiment of the presentinvention is to provide a light-emitting element which utilizes anexciplex and has high efficiency. A still further object of oneembodiment of the present invention is to provide a light-emittingelement which emits light from an exciplex and has high efficiency.

A yet still further object of one embodiment of the present invention isto provide a light-emitting device, a display device, an electronicdevice, and a lighting device each of which has high emission efficiencyby using any of the above light-emitting elements.

It is only necessary that at least one of the above-described objects beachieved in the present invention.

One embodiment of the present invention for achieving any of the aboveobjects is an organic compound with a triarylamine skeleton in which thethree aryl groups of the triarylamine skeleton are a p-biphenyl group, afluoren-2-yl group, and a phenyl group to which a dibenzofuranyl groupor a dibenzothiophenyl group is bonded.

Another embodiment of the present invention is an organic compoundrepresented by General Formula (G1).

In the formula, A represents any one of a dibenzofuranyl group and adibenzothiophenyl group, R¹ and R² separately represent any one ofhydrogen, an alkyl group having 1 to 6 carbon atoms, and a phenyl group.Note that when both R¹ and R² are phenyl groups, the phenyl groups maybe bonded to each other to form a spirofluorene skeleton.

A further embodiment of the present invention is an organic compoundrepresented by General Formula (G2).

In the formula, A represents any one of a dibenzofuranyl group and adibenzothiophenyl group, R¹ and R² separately represent any one ofhydrogen, an alkyl group having 1 to 6 carbon atoms, and a phenyl group.Note that when both R¹ and R² are phenyl groups, the phenyl groups maybe bonded to each other to form a spirofluorene skeleton.

A still further embodiment of the present invention is an organiccompound where the group represented by A in any of the above structuresis any one of groups represented by Structural Formulae (A-1) to (A-4).

A yet still further embodiment of the present invention is an organiccompound where the group represented by A in any of the above structuresis a group represented by Structural Formula (A-1) or (A-2).

A yet still further embodiment of the present invention is an organiccompound where the group represented by A in any of the above structuresis a group represented by Structural Formula (A-1).

A yet still further embodiment of the present invention is an organiccompound where R¹ and R² in any of the above structures separatelyrepresent any one of groups represented by Structural Formulae (R-1) to(R-12).

A yet still further embodiment of the present invention is an organiccompound where both R¹ and R² in any of the above structures are methylgroups.

A yet still further embodiment of the present invention is an organiccompound represented by Structural Formula (100).

A yet still further embodiment of the present invention is alight-emitting element that includes a pair of electrodes and a layercontaining an organic compound between the pair of electrodes. The layercontaining the organic compound contains any of the above organiccompounds.

A yet still further embodiment of the present invention is alight-emitting element that includes a pair of electrodes and a layercontaining an organic compound between the pair of electrodes. The layercontaining the organic compound includes at least a light-emittinglayer. The light-emitting layer contains any of the above organiccompounds.

A yet still further embodiment of the present invention is alight-emitting element that includes a pair of electrodes and a layercontaining an organic compound between the pair of electrodes. The layercontaining the organic compound includes at least a light-emittinglayer. The light-emitting layer contains at least a first organiccompound and a second organic compound. The first organic compound hasan electron-transport property. The second organic compound is any ofthe above organic compounds.

A yet still further embodiment of the present invention is alight-emitting element that includes a pair of electrodes and a layercontaining an organic compound between the pair of electrodes. The layercontaining the organic compound includes at least a light-emittinglayer. The light-emitting layer contains at least a first organiccompound, a second organic compound, and a phosphorescent substance. Thefirst organic compound has an electron-transport property. The secondorganic compound is any of the above organic compounds.

A yet still further embodiment of the present invention is alight-emitting element with any of the above structures, in which thefirst organic compound and the second organic compound form an exciplex.

A yet still further embodiment of the present invention is alight-emitting element with the above structure, in which tripletexcitation energy of each of the first organic compound and the secondorganic compound is higher than energy equivalent to a wavelength oflight emitted by the exciplex formed by the first organic compound andthe second organic compound (in other words, light energy of lightemitted by the exciplex formed by the first organic compound and thesecond organic compound).

A yet still further embodiment of the present invention is a lightingdevice which includes a light-emitting element having any of theabove-described structures.

A yet still further embodiment of the present invention is alight-emitting device which includes a light-emitting element having anyof the above-described structures and a unit which controls thelight-emitting element.

A yet still further embodiment of the present invention is a displaydevice which includes a light-emitting element having any of theabove-described structures in a display portion and a unit whichcontrols the light-emitting element.

A yet still further embodiment of the present invention is an electronicdevice which includes a light-emitting element having any of theabove-described structures.

Note that the light-emitting device in this specification includes, inits category, an image display device using a light-emitting element.Further, the category of the light-emitting device in this specificationincludes a module in which a light-emitting element is provided with aconnector such as an anisotropic conductive film or a tape carrierpackage (TCP); a module having a TCP at the tip of which a printedwiring board is provided; and a module in which an IC (integratedcircuit) is directly mounted on a light-emitting element by a COG (chipon glass) method. Furthermore, the category includes a light-emittingdevice which is used in lighting equipment or the like.

In one embodiment of the present invention, a novel organic compoundwhich forms an exciplex that efficiently emits light can be provided. Inone embodiment of the present invention, a light-emitting element whichhas high emission efficiency can be provided. In one embodiment of thepresent invention, a light-emitting element which utilizes an exciplexand has high efficiency can be provided. In one embodiment of thepresent invention, a light-emitting element which emits light from anexciplex and has high efficiency can be provided. In one embodiment ofthe present invention, a phosphorescent light-emitting element whichemits light via energy transfer from an exciplex and has high efficiencycan be provided.

In one embodiment of the present invention, a light-emitting device, adisplay device, an electronic device, and a lighting device each havinghigh emission efficiency can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are conceptual diagrams of light-emitting elements.

FIGS. 2A and 2B are conceptual diagrams of an active matrixlight-emitting device.

FIGS. 3A and 3B are conceptual diagrams of active matrix light-emittingdevices.

FIG. 4 is a conceptual diagram of an active matrix light-emittingdevice.

FIGS. 5A and 5B are conceptual diagrams of a passive matrixlight-emitting device.

FIGS. 6A and 6B are conceptual diagrams of a lighting device.

FIGS. 7A, 7B1, 7B2, 7C, and 7D illustrate electronic devices.

FIG. 8 illustrates an electronic device.

FIG. 9 illustrates a lighting device.

FIG. 10 illustrates a lighting device.

FIG. 11 illustrates in-vehicle display devices and lighting devices.

FIGS. 12A to 12C illustrate an electronic device.

FIGS. 13A and 13B are NMR charts of FrBBiF-II.

FIGS. 14A and 14B show absorption spectra and emission spectra ofFrBBiF-II.

FIG. 15 shows results of LC/MS analysis of FrBBiF-II.

FIG. 16 shows current density-luminance characteristics of alight-emitting element 1 and a comparative light-emitting element 1.

FIG. 17 shows luminance-current efficiency characteristics of alight-emitting element 1 and a comparative light-emitting element 1.

FIG. 18 shows voltage-luminance characteristics of a light-emittingelement 1 and a comparative light-emitting element 1.

FIG. 19 shows luminance-power efficiency characteristics of alight-emitting element 1 and a comparative light-emitting element 1.

FIG. 20 shows luminance-external quantum efficiency characteristics of alight-emitting element 1 and a comparative light-emitting element 1.

FIG. 21 shows emission spectra of a light-emitting element 1 and acomparative light-emitting element 1.

FIG. 22 shows current density-luminance characteristics of alight-emitting element 2.

FIG. 23 shows luminance-current efficiency characteristics of alight-emitting element 2.

FIG. 24 shows voltage-luminance characteristics of a light-emittingelement 2.

FIG. 25 shows luminance-power efficiency characteristics of alight-emitting element 2.

FIG. 26 shows luminance-external quantum efficiency characteristics of alight-emitting element 2.

FIG. 27 shows an emission spectrum of a light-emitting element 2.

FIG. 28 shows time dependence of normalized luminance of alight-emitting element 2.

FIG. 29 shows luminance-current efficiency characteristics of alight-emitting element 3.

FIG. 30 shows voltage-luminance characteristics of a light-emittingelement 3.

FIG. 31 shows voltage-current characteristics of a light-emittingelement 3.

FIG. 32 shows an emission spectrum of a light-emitting element 3.

FIG. 33 shows time dependence of normalized luminance of alight-emitting element 3.

FIGS. 34A and 34B are NMR charts of ThBBiF.

FIG. 35 shows results of LC/MS analysis of ThBBiF.

FIGS. 36A and 36B show absorption spectra and emission spectra ofThBBiF.

FIG. 37 shows current density-luminance characteristics of alight-emitting element 4.

FIG. 38 shows luminance-current efficiency characteristics of alight-emitting element 4.

FIG. 39 shows voltage-luminance characteristics of a light-emittingelement 4.

FIG. 40 shows luminance-power efficiency characteristics of alight-emitting element 4.

FIG. 41 shows luminance-external quantum efficiency characteristics of alight-emitting element 4.

FIG. 42 shows an emission spectrum of a light-emitting element 4.

FIG. 43 shows time dependence of normalized luminance of alight-emitting element 4.

FIG. 44 shows current density-luminance characteristics of alight-emitting element 5.

FIG. 45 shows luminance-current efficiency characteristics of alight-emitting element 5.

FIG. 46 shows voltage-luminance characteristics of a light-emittingelement 5.

FIG. 47 shows luminance-power efficiency characteristics of alight-emitting element 5.

FIG. 48 shows luminance-external quantum efficiency characteristics of alight-emitting element 5.

FIG. 49 shows an emission spectrum of a light-emitting element 5.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be explained below withreference to the drawings. Note that the present invention is notlimited to the description below, and it is easily understood by thoseskilled in the art that various changes and modifications can be madewithout departing from the spirit and scope of the present invention.Accordingly, the present invention should not be interpreted as beinglimited to the content of the embodiments below.

Embodiment 1

As one embodiment of the present invention for achieving any of theabove objects, a novel organic compound with a triarylamine skeleton inwhich the three aryl groups of the triarylamine skeleton are ap-biphenyl group, a fluoren-2-yl group, and a phenyl group to which adibenzofuranyl group or a dibenzothiophenyl group is bonded will bedescribed in this embodiment.

Three aryl groups are bonded to nitrogen of a triarylamine skeleton. Inan organic compound in this embodiment, the three aryl groups are ap-biphenyl group, a fluoren-2-yl group, and a phenyl group to which adibenzofuranyl group or a dibenzothiophenyl group is bonded.

The carbon at the 9-position of the fluoren-2-yl group may have one ortwo substituents, which are separately any one of an alkyl group having1 to 6 carbon atoms and a phenyl group. When both of the substituentsare phenyl groups, the phenyl groups may be bonded to each other to forma spirofluorene skeleton.

Further, the dibenzofuranyl group or the dibenzothiophenyl group may bebonded to the ortho-position, the meta-position, or the para-position ofthe phenyl group but is preferably bonded to the para-position in termsof reliability. The dibenzofuranyl group or the dibenzothiophenyl groupis preferably bonded to the phenyl group at the 4-position or the2-position of the dibenzofuranyl group or the dibenzothiophenyl group interms of synthesis, and the dibenzofuranyl group or thedibenzothiophenyl group is further preferably bonded to the phenyl groupat the 4-position of the dibenzofuranyl group or the dibenzothiophenylgroup.

Note that in the organic compound described in this embodiment, each ofthe p-biphenyl group and the fluoren-2-yl group, which are the arylgroups bonded to nitrogen of the amine, has a biphenyl skeleton and doesnot have a terphenyl skeleton. The fluorene skeleton can be regarded asa bridged biphenyl skeleton.

Here, if the biphenyl skeleton is replaced with a terphenyl group or agroup including three or more benzene skeletons, the triplet excitationlevel is lowered, which inhibits efficient light emission from anexciplex. Further, if a phenyl group is bonded as a substituent to aposition other than the 9-position of the fluoren-2-yl group, aterphenyl skeleton is formed, which is also problematic. Note that ap-biphenyl group and a fluoren-2-yl group are preferable as substituentsin terms of reliability.

The other aryl group bonded to nitrogen of the amine, i.e., the phenylgroup to which a dibenzofuranyl group or a dibenzothiophenyl group isbonded, also plays an important role. If the phenyl group is replacedwith a group having more benzene skeletons than a phenyl group, e.g., abiphenyl group, the biphenyl group and one of the benzene rings of thedibenzofuranyl group or the dibenzothiophenyl group form a terphenylskeleton, which also inhibits efficient light emission from an exciplex.

The above-described organic compound can be more specificallyrepresented by General Formula (G1).

In General Formula (G1), the phenylene group to which the grouprepresented by A is bonded is preferably a p-phenylene group. In otherwords, a structure represented by General Formula (G2) is preferable.

In General Formula (G1) or (G2), A represents any one of adibenzofuranyl group and a dibenzothiophenyl group.

The dibenzofuranyl group or the dibenzothiophenyl group is preferablyany one of groups represented by Structural Formulae (A-1) to (A-4), andfurther preferably a group represented by Structural Formula (A-1) or(A-2).

R¹ and R² separately represent any one of hydrogen, an alkyl grouphaving 1 to 6 carbon atoms, and a phenyl group. When both R¹ and R² arephenyl groups, the phenyl groups may be bonded to each other to form aspirofluorene skeleton. Specific examples of R¹ and R² include groupsrepresented by Structural Formulae (R-1) to (R-12).

Specific examples of an organic compound having the above structure arerepresented by Structural Formulae (100) to (109) and (200) to (209).Note that an organic compound of one embodiment of the present inventionis not limited to the examples below.

A method for synthesizing the above organic compound is described. Asshown in a reaction scheme below, the organic compound represented byGeneral Formula (G1) can be obtained by coupling of an aryl compound(a1) having a halogen group and an aryl compound (a2) having an amine.

In the synthesis scheme, A represents any one of a dibenzofuranyl groupand a dibenzothiophenyl group, R¹ and R² separately represent any one ofhydrogen, an alkyl group having 1 to 6 carbon atoms, and a phenyl group.Note that when both R¹ and R² are phenyl groups, the phenyl groups maybe bonded to each other to form a spirofluorene skeleton. X¹ representsa halogen, preferably bromine or iodine, which has high reactivity, morepreferably iodine.

In the synthesis scheme above, there are a variety of reactionconditions for the coupling reaction of the aryl compound having ahalogen group and the aryl compound having an amine (secondary arylaminecompound); for example, a synthesis method using a metal catalyst in thepresence of a base can be applied.

The case where a Hartwig-Buchwald reaction is used in the synthesisscheme is described. A palladium catalyst can be used as the metalcatalyst, and a mixture of a palladium complex and a ligand thereof canbe used as the palladium catalyst. As the palladium complex,bis(dibenzylideneacetone)palladium(0), palladium(II) acetate, and thelike are given. Examples of the ligand include tri(tert-butyl)phosphine,tri(n-hexyl)phosphine, tricyclohexylphosphine,1,1′-bis(diphenylphosphino)ferrocene (abbreviation: DPPF),di(1-adamantyl)-n-butylphosphine, tris(2,6-dimethoxyphenyl)phosphine,and the like. Examples of a substance which can be used as the baseinclude organic bases such as sodium tert-butoxide, inorganic bases suchas potassium carbonate, tripotassium phosphate, cesium carbonate, andthe like. In addition, this reaction is preferably performed in asolution, and examples of the solvent that can be used are toluene,xylene, benzene, mesitylene, and the like. However, the catalyst,ligand, base, and solvent which can be used are not limited to theseexamples. In addition, the reaction is preferably performed under aninert atmosphere of nitrogen, argon, or the like.

The case where an Ullmann reaction is used in the synthesis scheme isdescribed. A copper catalyst can be used as the metal catalyst, andcopper(I) iodide and copper(II) acetate are given as the coppercatalyst. As an example of a substance which can be used for the base,an inorganic base such as potassium carbonate is given. The reaction ispreferably performed in a solution, and examples of the solvent that canbe used are 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)pyrimidinone(abbreviation: DMPU), toluene, xylene, benzene, mesitylene, and thelike. However, the catalyst, base, and solvent which can be used are notlimited to these examples. In addition, the reaction is preferablyperformed under an inert atmosphere of nitrogen, argon, or the like.

Note that a solvent having a high boiling point such as DMPU, xylene, ormesitylene is preferably used because, in an Ullmann reaction, a targetsubstance can be obtained in a shorter time and at a higher yield whenthe reaction temperature is 100° C. or higher. A reaction temperaturehigher than 150° C. is further preferred and accordingly DMPU ormesitylene is more preferably used.

Through the above-described steps, the organic compound described inthis embodiment can be synthesized.

By the use of the organic compound described in this embodiment, alight-emitting element using an exciplex as an emission center can haveextremely high external quantum efficiency. A light-emitting element inwhich the organic compound is used as one of substances forming anexciplex that is an emission center can be driven at low voltage.Accordingly, the light-emitting element can have extremely high powerefficiency. Note that the organic compound described in this embodimenthas a hole-transport property, and thus it can be favorably used as amaterial included in a hole-transport layer.

A light-emitting element in which the exciplex is used as a substancehaving a function of transferring energy to a phosphorescent substancecan also have high external quantum efficiency. Such a light-emittingelement can be driven at low voltage and thus can have extremely highpower efficiency.

Embodiment 2

In this embodiment, description is given of a structure of alight-emitting element in which the organic compound described inEmbodiment 1 is used as one of two kinds of substances forming anexciplex.

As a method for converting a triplet excited state into light emission,there are a method utilizing phosphorescence, which is direct emissionfrom a triplet excited state, and a method utilizing delayedfluorescence, which is light emitted from a singlet excited state aftera triplet excited state is turned into a singlet excited state viareverse intersystem crossing.

A structure of a light-emitting element that uses a phosphorescentmaterial and emits light with extremely high efficiency has beenactually reported, which proves advantages of the utilization of atriplet excited state for light emission.

Some degree of success in a light-emitting element using a delayedfluorescence material has been achieved in recent years. However, asubstance emitting delayed fluorescence with relatively high efficiencyhas an extremely rare state where a singlet excited state and a tripletexcited state are close to each other and accordingly has a uniquemolecular structure; thus, the kind of such a substance is stilllimited.

It has been reported that an exciplex (also called excited complex) is acomplex in an excited state which is formed by two kinds of moleculesdue to charge-transfer interaction and that the singlet excited stateand the triplet excited state of an exciplex are close to each other inmany cases.

Therefore, an exciplex readily emits delayed fluorescence even at roomtemperature and might allow a fluorescent light-emitting element to havehigh efficiency. Light emitted by an exciplex has a wavelengthequivalent to a difference between a shallower HOMO level and a deeperLUMO level of the two kinds of substances that form the complex. Thus,light with a desired wavelength can be obtained relatively easily byselection of substances forming an exciplex.

However, positive use of light emission from an exciplex is still underinvestigation. There are few guidelines for selecting substances toachieve high emission efficiency, and without any guideline, a favorablelight-emitting element will never be provided.

In view of the above, in this embodiment, a structure of alight-emitting element in which an exciplex is used as an emissioncenter and which emits light with high efficiency is described.

A light-emitting element in this embodiment includes a layer containingan organic compound (the layer may also contain an inorganic compound)between a pair of electrodes, and the layer containing an organiccompound at least includes a light-emitting layer. The light-emittinglayer contains a first organic compound with an electron-transportproperty and a second organic compound with a hole-transport property.

A combination of the first organic compound and the second organiccompound forms an exciplex when they are excited by a current or when acurrent flows therein. To form an exciplex, the HOMO level and LUMOlevel of the first organic compound are preferably positioned deeperthan the HOMO level and LUMO level of the second organic compound,respectively.

The formation process of the exciplex is considered to be roughlyclassified into the following two processes.

One formation process is the process in which an exciplex is formed bythe first organic compound with an electron-transport property and thesecond organic compound with a hole-transport property which are in thestate of having carriers (cation or anion).

The other formation process is an elementary process in which one of thefirst organic compound with an electron-transport property and thesecond organic compound with a hole-transport property forms a singletexciton and then the singlet exciton interacts with the other in theground state to form an exciplex.

The exciplex in this embodiment may be formed by either process.

Here, when the organic compound described in Embodiment 1 is used as thesecond organic compound with a hole-transport property, efficient lightemission from the exciplex can be obtained.

It is preferable that triplet excitation energy of each of the firstorganic compound and the second organic compound (energy equivalent to adifference between a triplet excited level and a singlet excited level)be higher than triplet excitation energy of the exciplex. This isbecause when the triplet excitation energy of each of the first organiccompound and the second organic compound is lower than that of theexciplex, the triplet excitation energy of the exciplex is transferred,which inhibits efficient light emission.

Note that triplet excitation energy of an exciplex, whose singletexcited state and triplet excited state has a small energy difference,can be considered equivalent to the emission wavelength of the exciplex.

As the first organic compound with an electron-transport property, anelectron-transport material having an electron mobility of 10⁻⁶ cm²/Vsor higher can be used mainly. Specifically, a π-electron deficientheteroaromatic compound such as a nitrogen-containing heteroaromaticcompound is preferable, and for example, the following compounds can beused: heterocyclic compounds having polyazole skeletons, such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ),1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation:CO11), 2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), and2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole(abbreviation: mDBTBIm-II); heterocyclic compounds having quinoxalineskeletons or dibenzoquinoxaline skeletons, such as2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II), 7-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline(abbreviation: 7mDBTPDBq-II),6-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:6mDBTPDBq-II),2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II), and2-[3′-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mCzBPDBq); heterocyclic compounds having diazineskeletons (pyrimidine skeletons or pyrazine skeletons), such as4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine (abbreviation:4,6mPnP2Pm), 4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine(abbreviation: 4,6mDBTP2Pm-II), and4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation:4,6mCzP2Pm); and heterocyclic compounds having pyridine skeletons, suchas 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation: 35DCzPPy),1,3,5-tri[3-(3-pyridyl)phenyl]benzene (abbreviation: TmPyPB), and3,3′,5,5′-tetra[(m-pyridyl)-phen-3-yl]biphenyl (abbreviation: BP4mPy).Among the above-described compounds, the heterocyclic compounds havingquinoxaline skeletons or dibenzoquinoxaline skeletons, the heterocycliccompounds having diazine skeletons, and the heterocyclic compoundshaving pyridine skeletons have high reliability and can be preferablyused. The following can also be given as the first organic compound:triaryl phosphine oxides such as phenyl-di(1-pyrenyl)phosphine oxide(abbreviation: POPy₂), Spiro-9,9′-bifluoren-2-yl-diphenylphosphine oxide(abbreviation: SPPO1), 2,8-bis(diphenylphosphoryl)dibenzo[b,d]thiophene(abbreviation: PPT), and3-(diphenylphosphoryl)-9-[4-(diphenylphosphoryl)phenyl]-9H-carbazole(abbreviation: PPO21); and triaryl borane such astris[2,4,6-trimethyl-3-(3-pyridyl)phenyl]borane (abbreviation: 3TPYMB).Note that a heterocyclic compound with a diazine skeleton, specificallya heterocyclic compound with a pyrimidine skeleton, is preferably used,in which case light can be emitted with higher efficiency.

An exciplex formed by the first organic compound as described above andthe second organic compound that is the organic compound in Embodiment 1can emit light with extremely high efficiency, and accordingly, thelight-emitting element in this embodiment can emit light with highefficiency. Although the theoretical limit of external quantumefficiency of a fluorescent light-emitting element is generallyconsidered to be 5% to 7% when it is not designed to enhance extractionefficiency, a light-emitting element having external quantum efficiencyhigher than the theoretical limit can be easily provided by the use ofthe structure of the light-emitting element in this embodiment.

As described above, because the emission wavelength of an exciplex isequivalent to a difference between a shallower HOMO level and a deeperLUMO level of the first and second organic compounds, a light-emittingelement emitting light with a desired wavelength can be easily providedby selection of substances each of which has an appropriate level.

In this manner, the structure in this embodiment makes it possible toprovide a high-efficiency light-emitting element in which a tripletexcited state can be converted into light emission. Besides,light-emitting elements with such characteristics can be providedwithout severe limitation on their emission wavelengths.

Note that a light-emitting layer of a light-emitting element with thestructure described in this embodiment may be added with a fluorescentsubstance so that light emission from the fluorescent substance isprovided. Because the light-emitting element uses an exciplex capable ofconverting a triplet excited state into a singlet excited state as asubstance having a function of transferring energy, the fluorescentsubstance can emit light with high efficiency. Moreover, thelight-emitting element can have a long lifetime because light emissionis obtained from the fluorescent substance, which has stability.

Embodiment 3

In this embodiment, description is given of the light-emitting elementin Embodiment 2 in which the light-emitting layer further contains aphosphorescent substance and which emits light from the phosphorescentsubstance. The light-emitting element in this embodiment has the samestructure as the light-emitting element in Embodiment 2 except that thelight-emitting layer contains the phosphorescent substance. Descriptionof the common structures and materials is not repeated. Thecorresponding description in Embodiment 2 is to be referred to.

Because the light-emitting layer contains the phosphorescent substance,efficient energy transfer from an exciplex to the phosphorescentsubstance can be performed.

Here, to achieve high emission efficiency of a light-emitting elementthat uses a phosphorescent substance, energy transfer between the hostmaterial and the phosphorescent substance will be considered. Carrierrecombination occurs in both the host material and the phosphorescentsubstance; thus, efficient energy transfer from the host material to thephosphorescent substance is necessary to increase emission efficiency.

As mechanisms of the energy transfer from the host material to thephosphorescent substance, two mechanisms have been proposed: one isDexter mechanism, and the other is Förster mechanism. Each mechanism isdescribed below. Here, a molecule providing excitation energy isreferred to as a host molecule, while a molecule receiving theexcitation energy is referred to as a guest molecule.

<<Förster Mechanism (Dipole-Dipole Interaction)>>

Förster mechanism (also referred to as Förster resonance energytransfer) does not require direct contact between molecules for energytransfer. Through a resonant phenomenon of dipolar oscillation between ahost molecule and a guest molecule, energy transfer occurs. By theresonant phenomenon of dipolar oscillation, the host molecule providesenergy to the guest molecule, and thus, the host molecule returns to aground state and the guest molecule reaches an excited state. The rateconstant k_(h*→g) of Förster mechanism is expressed by Formula (1).

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack & \; \\{k_{h^{*}\rightarrow g} = {\frac{9000\; c^{4}K^{2}{\varphi ln10}}{128\; \pi^{5}n^{4}N\; \pi \; R^{6}}{\int{\frac{{f_{h}^{\prime}(v)}{ɛ_{g}(v)}}{v^{4}}{v}}}}} & (1)\end{matrix}$

In Formula (1), ν denotes a frequency, f′_(h)(ν) denotes a normalizedemission spectrum of a host molecule (a fluorescence spectrum in energytransfer from a singlet excited state, and a phosphorescence spectrum inenergy transfer from a triplet excited state), ε_(g)(ν) denotes a molarabsorption coefficient of a guest molecule, N denotes Avogadro's number,n denotes a refractive index of a medium, R denotes an intermoleculardistance between the host molecule and the guest molecule, τ denotes ameasured lifetime of an excited state (fluorescence lifetime orphosphorescence lifetime), c denotes the speed of light, φ denotes aluminescence quantum yield (a fluorescence quantum yield in energytransfer from a singlet excited state, and a phosphorescence quantumyield in energy transfer from a triplet excited state), and K² denotes acoefficient (0 to 4) of orientation of a transition dipole momentbetween the host molecule and the guest molecule. Note that K²=⅔ inrandom orientation.

<<Dexter Mechanism (Electron Exchange Interaction)>>

In Dexter mechanism (also referred to as Dexter electron transfer), ahost molecule and a guest molecule are close to a contact effectiverange where their orbitals can overlap, and the host molecule in anexcited state and the guest molecule in a ground state exchange theirelectrons, which leads to energy transfer. The rate constant k_(h*→g) ofDexter mechanism is expressed by Formula (2).

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack & \; \\{k_{h^{*}\rightarrow g} = {\left( \frac{2\pi}{h} \right)K^{2}{\exp \left( {- \frac{2R}{L}} \right)}{\int{{f_{h}^{\prime}(v)}{ɛ_{g}^{\prime}(v)}{v}}}}} & (2)\end{matrix}$

In Formula (2), h denotes a Planck constant, K denotes a constant havingan energy dimension, ν denotes a frequency, f′_(h)(ν) denotes anormalized emission spectrum of a host molecule (a fluorescence spectrumin energy transfer from a singlet excited state, and a phosphorescencespectrum in energy transfer from a triplet excited state), ε′_(g)(ν)denotes a normalized absorption spectrum of a guest molecule, L denotesan effective molecular radius, and R denotes an intermolecular distancebetween the host molecule and the guest molecule.

Here, the efficiency of energy transfer from the host molecule to theguest molecule (energy transfer efficiency Φ_(ET)) is expressed byFormula (3). In the formula, k_(r) denotes a rate constant of alight-emission process (fluorescence in energy transfer from a singletexcited state, and phosphorescence in energy transfer from a tripletexcited state), k_(n) denotes a rate constant of a non-light-emissionprocess (thermal deactivation or intersystem crossing), and τ denotes ameasured lifetime of an excited state.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack & \; \\{\Phi_{ET} = {\frac{k_{h^{*}\rightarrow g}}{k_{r} + k_{n} + k_{h^{*}\rightarrow g}} = \frac{k_{h^{*}\rightarrow g}}{\left( \frac{1}{\tau} \right) + k_{h^{*}\rightarrow g}}}} & (3)\end{matrix}$

First, according to Formula (3), it is understood that the energytransfer efficiency Φ_(ET) can be increased by significantly increasingthe rate constant k_(h*→g) of energy transfer as compared with anothercompeting rate constant k_(r)+k_(n) (=1/τ). Then, in order to increasethe rate constant k_(h*→g) of energy transfer, based on Formulae (1) and(2), in Förster mechanism and Dexter mechanism, it is preferable that anemission spectrum of a host molecule (a fluorescence spectrum in energytransfer from a singlet excited state, and a phosphorescence spectrum inenergy transfer from a triplet excited state) has a large overlap withan absorption spectrum of a guest molecule.

Here, a longest-wavelength-side (lowest-energy-side) absorption band inthe absorption spectrum of the guest molecule is important inconsidering the overlap between the emission spectrum of the hostmolecule and the absorption spectrum of the guest molecule.

In this embodiment, a phosphorescent substance is used as the guestmaterial. In an absorption spectrum of the phosphorescent substance, anabsorption band that is considered to contribute to light emission mostgreatly is at an absorption wavelength corresponding to directtransition from a ground state to a triplet excitation state and avicinity of the absorption wavelength, which is on the longestwavelength side. Therefore, it is considered preferable that theemission spectrum (a fluorescence spectrum and a phosphorescencespectrum) of the host material overlap with the absorption band on thelongest wavelength side in the absorption spectrum of the phosphorescentsubstance.

For example, most organometallic complexes, especially light-emittingiridium complexes, have a broad absorption band around 500 nm to 600 nmas the absorption band on the longest wavelength side. This absorptionband is mainly based on a triplet MLCT (metal to ligand charge transfer)transition. Note that it is considered that the absorption band alsoincludes absorptions based on a triplet π-π* transition and a singletMLCT transition, and that these absorptions overlap each other to form abroad absorption band on the longest wavelength side in the absorptionspectrum. Therefore, when an organometallic complex (especially iridiumcomplex) is used as the guest material, it is preferable to make thebroad absorption band on the longest wavelength side have a largeoverlap with the emission spectrum of the host material as describedabove.

Here, first, energy transfer from a host material in a triplet excitedstate will be considered. From the above-described discussion, it ispreferable that, in energy transfer from a triplet excited state, thephosphorescence spectrum of the host material and the absorption band onthe longest wavelength side of the guest material have a large overlap.

However, a question here is energy transfer from the host molecule inthe singlet excited state. In order to efficiently perform not onlyenergy transfer from the triplet excited state but also energy transferfrom the singlet excited state, it is clear from the above-describeddiscussion that the host material needs to be designed such that notonly its phosphorescence spectrum but also its fluorescence spectrumoverlaps with the absorption band on the longest wavelength side of theguest material. In other words, unless the host material is designed soas to have its fluorescence spectrum in a position similar to that ofits phosphorescence spectrum, it is not possible to achieve efficientenergy transfer from the host material in both the singlet excited stateand the triplet excited state.

However, in general, the S₁ level differs greatly from the T₁ level (S₁level>T₁ level); therefore, the fluorescence emission wavelength alsodiffers greatly from the phosphorescence emission wavelength(fluorescence emission wavelength<phosphorescence emission wavelength).For example, 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP), which iscommonly used in a light-emitting element containing a phosphorescentsubstance, has a phosphorescence spectrum around 500 nm and has afluorescence spectrum around 400 nm, which are largely different byapproximately 100 nm. This example also shows that it is extremelydifficult to design a host material so as to have its fluorescencespectrum in a position similar to that of its phosphorescence spectrum.

Also, since the S₁ level is higher than the T₁ level, the T₁ level of ahost material whose fluorescence spectrum corresponds to a wavelengthclose to an absorption spectrum of a guest material on the longestwavelength side is lower than the T₁ level of the guest material.

Thus, an exciplex is utilized as in this embodiment, so that theefficiency of energy transfer can be enhanced.

A fluorescence spectrum of the exciplex is on a longer wavelength sidethan a fluorescence spectrum of the first organic compound alone or thesecond organic compound alone. Therefore, energy transfer from a singletexcited state can be maximized while the T₁ levels of the first organiccompound and the second organic compound are kept higher than the T₁level of the guest material. In addition, the exciplex is in a statewhere the T₁ level and the S₁ level are close to each other; therefore,the fluorescence spectrum and the phosphorescence spectrum exist atsubstantially the same position. Accordingly, both the fluorescencespectrum and the phosphorescence spectrum of the exciplex can have alarge overlap with an absorption corresponding to transition of theguest molecule from the singlet ground state to the triplet excitedstate (a broad absorption band of the guest molecule existing on thelongest wavelength side in the absorption spectrum), and thus alight-emitting element having high energy transfer efficiency can beobtained.

Carrier balance can be controlled by adjusting the mixture ratio of thefirst organic compound with an electron-transport property to the secondorganic compound with a hole-transport property. Specifically, the ratioof the first organic compound to the second organic compound (oradditive) is preferably from 1:9 to 9:1. Note that in that case, thefollowing structure may be employed: a light-emitting layer in which onekind of a phosphorescent substance is dispersed is divided into twolayers, and the two layers have different mixture ratios of the firstorganic compound to the second organic compound. With this structure,the carrier balance in the light-emitting element can be optimized, sothat the lifetime of the light-emitting element can be improved.Furthermore, in this case, one of the light-emitting layers may be ahole-transport layer and the other of the light-emitting layers may bean electron-transport layer.

A material that can be used as the phosphorescent substance is notparticularly limited. Examples of a blue phosphorescent substanceinclude an organometallic iridium complex having a 4H-triazole skeleton,such astris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-κN2]phenyl-κC}iridium(III)(abbreviation: Ir(mpptz-dmp)₃),tris(5-methyl-3,4-diphenyl-4H-1,2,4-triazolato)iridium(III)(abbreviation: Ir(Mptz)₃), ortris[4-(3-biphenyl)-5-isopropyl-3-phenyl-4H-1,2,4-triazolato]iridium(III)(abbreviation: Ir(iPrptz-3b)₃); an organometallic iridium complex havinga 1H-triazole skeleton, such astris[3-methyl-1-(2-methylphenyl)-5-phenyl-1H-1,2,4-triazolato]iridium(III)(abbreviation: Ir(Mptzl-mp)₃) ortris(1-methyl-5-phenyl-3-propyl-1H-1,2,4-triazolato)iridium(III)(abbreviation: Ir(Prptzl-Me)₃); an organometallic iridium complex havingan imidazole skeleton, such asfac-tris[1-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole]iridium(III)(abbreviation: Ir(iPrpmi)₃) ortris[3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f]phenanthridinato]iridium(III)(abbreviation: Ir(dmpimpt-Me)₃); and an organometallic iridium complexin which a phenylpyridine derivative having an electron-withdrawinggroup is a ligand, such asbis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)tetrakis(1-pyrazolyl)borate (abbreviation: FIr6),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III) picolinate(abbreviation: Flrpic),bis{2-[3′,5′-bis(trifluoromethyl)phenyl]pyridinato-N,C^(2′)}iridium(III)picolinate (abbreviation: Ir(CF₃ppy)₂(pic)), orbis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)acetylacetonate (abbreviation: FIr(acac)). Note that an organometalliciridium complex having a 4H-triazole skeleton has excellent reliabilityand emission efficiency and thus is especially preferable. Examples of agreen phosphorescent substance include an organometallic iridium complexhaving a pyrimidine skeleton, such astris(4-methyl-6-phenylpyrimidinato)iridium(III) (abbreviation:Ir(mppm)₃), tris(4-t-butyl-6-phenylpyrimidinato)iridium(III)(abbreviation: Ir(tBuppm)₃),(acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III)(abbreviation: Ir(mppm)₂(acac)),bis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III)(abbreviation: Ir(tBuppm)₂(acac)),(acetylacetonato)bis[6-(2-norbornyl)-4-phenylpyrimidinato]iridium(III)(abbreviation: Ir(nbppm)₂(acac)),(acetylacetonato)bis[5-methyl-6-(2-methylphenyl)-4-phenylpyrimidinato]iridium(III)(abbreviation: Ir(mpmppm)₂(acac)), or(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)(abbreviation: Ir(dppm)₂(acac)); an organometallic iridium complexhaving a pyrazine skeleton, such as(acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III)(abbreviation: Ir(mppr-Me)₂(acac)) or(acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III)(abbreviation: Ir(mppr-iPr)₂(acac)); an organometallic iridium complexhaving a pyridine skeleton, such astris(2-phenylpyridinato-N,C^(2′))iridium(III) (abbreviation: Ir(ppy)₃),bis(2-phenylpyridinato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: Ir(ppy)₂(acac)), bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation: Ir(bzq)₂(acac)),tris(benzo[h]quinolinato)iridium(III) (abbreviation: Ir(bzq)₃),tris(2-phenylquinolinato-N,C^(2′))iridium(III) (abbreviation: Ir(pq)₃),or bis(2-phenylquinolinato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: Ir(pq)₂(acac)); and a rare earth metal complex such astris(acetylacetonato)(monophenanthroline)terbium(III) (abbreviation:Tb(acac)₃(Phen)). Note that an organometallic iridium complex having apyrimidine skeleton has distinctively high reliability and emissionefficiency and thus is especially preferable. Examples of a redphosphorescent substance include an organometallic iridium complexhaving a pyrimidine skeleton, such as(diisobutyrylmethanato)bis[4,6-bis(3-methylphenyl)pyrimidinato]iridium(III)(abbreviation: Ir(5mdppm)₂(dibm)),bis[4,6-bis(3-methylphenyl)pyrimidinato](dipivaloylmethanato)iridium(III)(abbreviation: Ir(5mdppm)₂(dpm)), orbis[4,6-di(naphthalen-1-yl)pyrimidinato](dipivaloylmethanato)iridium(III)(abbreviation: Ir(d1npm)₂(dpm)); an organometallic iridium complexhaving a pyrazine skeleton, such as(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: Ir(tppr)₂(acac)),bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: Ir(tppr)₂(dpm)),(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)); an organometallic iridium complexhaving a pyridine skeleton, such astris(1-phenylisoquinolinato-N,C^(2′))iridium(III) (abbreviation:Ir(piq)₃) orbis(1-phenylisoquinolinato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(piq)₂(acac)); a platinum complex such as2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II)(abbreviation: PtOEP); and a rare earth metal complex such astris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III)(abbreviation: Eu(DBM)₃(Phen)) ortris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III)(abbreviation: Eu(TTA)₃(Phen)). Note that an organometallic iridiumcomplex having a pyrimidine skeleton has distinctively high reliabilityand emission efficiency and thus is especially preferable. Further,because an organometallic iridium complex having a pyrazine skeleton canprovide red light emission with favorable chromaticity, the use of theorganometallic iridium complex in a white light-emitting elementimproves a color rendering property of the white light-emitting element.Note that an organic compound with a benzofuropyrimidine skeleton alsoemits light in blue to ultraviolet regions and thus can be used as anemission center material. A compound with a benzofuropyrimidine skeletonmay also be used.

The material that can be used as the phosphorescent substance may alsobe selected from various other substances instead of the substancesgiven above.

A light-emitting element with the above structure has high energytransfer efficiency and thus can have high external quantum efficiency.A phosphorescent light-emitting element that emits light by utilizingenergy transfer from an exciplex can be driven at low voltage.Accordingly, the light-emitting element in this embodiment can haveextremely high power efficiency.

Embodiment 4

In this embodiment, a detailed example of the structure of thelight-emitting element described in Embodiment 2 or 3 will be describedbelow with reference to FIGS. 1A and 1B.

In FIG. 1A, the light-emitting element includes a first electrode 101, asecond electrode 102, and a layer 103 containing an organic compound andprovided between the first electrode 101 and the second electrode 102.Note that in this embodiment, the first electrode 101 functions as ananode and the second electrode 102 functions as a cathode. In otherwords, when voltage is applied between the first electrode 101 and thesecond electrode 102 so that the potential of the first electrode 101 ishigher than that of the second electrode 102, light emission can beobtained. The layer 103 containing an organic compound at least includesa light-emitting layer 113. A hole-injection layer 111, a hole-transportlayer 112, an electron-transport layer 114, and an electron-injectionlayer 115 which are illustrated in FIG. 1A are merely examples and notnecessarily provided. A layer having any other function may also beprovided.

The first electrode 101 functions as the anode and is preferably formedusing any of metals, alloys, electrically conductive compounds with ahigh work function (specifically, a work function of 4.0 eV or more),mixtures thereof, and the like. Specific examples are indium oxide-tinoxide (ITO: indium tin oxide), indium oxide-tin oxide containing siliconor silicon oxide, indium oxide-zinc oxide, indium oxide containingtungsten oxide and zinc oxide (IWZO), and the like. Such conductivemetal oxide films are usually formed by a sputtering method, but mayalso be formed by application of a sol-gel method or the like. In anexample of the formation method, indium oxide-zinc oxide is deposited bya sputtering method using a target obtained by adding 1 wt % to 20 wt %of zinc oxide to indium oxide. Further, a film of indium oxidecontaining tungsten oxide and zinc oxide (IWZO) can be formed by asputtering method using a target in which tungsten oxide and zinc oxideare added to indium oxide at 0.5 wt % to 5 wt % and 0.1 wt % to 1 wt %,respectively. In addition, gold (Au), platinum (Pt), nickel (Ni),tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co),copper (Cu), palladium (Pd), a nitride of a metal material (such astitanium nitride), or the like can be used. Graphene can also be used.Note that when a composite material described later is used for a layerwhich is in contact with the first electrode 101 in the layer 103containing an organic compound, an electrode material can be selectedregardless of its work function.

There is no particular limitation on the stacked structure of the layer103 containing an organic compound as long as the light-emitting layer113 has the structure described in Embodiment 2 or 3. For example, inFIG. 1A, the layer 103 containing an organic compound can be formed bycombining a hole-injection layer, a hole-transport layer, thelight-emitting layer, an electron-transport layer, an electron-injectionlayer, a carrier-blocking layer, a charge-generation layer, and the likeas appropriate. In this embodiment, the layer 103 containing an organiccompound has a structure in which the hole-injection layer 111, thehole-transport layer 112, the light-emitting layer 113, theelectron-transport layer 114, and the electron-injection layer 115 arestacked in this order over the first electrode 101. Materials for thelayers are specifically given below.

The hole-injection layer 111 is a layer containing a substance having ahigh hole-injection property. Molybdenum oxide, vanadium oxide,ruthenium oxide, tungsten oxide, manganese oxide, or the like can beused. Alternatively, the hole-injection layer 111 can be formed using aphthalocyanine-based compound such as phthalocyanine (abbreviation:H₂Pc) or copper phthalocyanine (abbreviation: CuPc), an aromatic aminecompound such as4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB) orN,N′-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (abbreviation: DNTPD), a high molecular compound such aspoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS),or the like.

Alternatively, a composite material in which a material with ahole-transport property contains an acceptor substance can be used forthe hole-injection layer 111. Note that the use of such a material witha hole-transport property which contains an acceptor substance enablesselection of a material used to four an electrode regardless of its workfunction. In other words, besides a material having a high workfunction, a material having a low work function can also be used for thefirst electrode 101. As the acceptor substance,7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil, and the like can be given. In addition, atransition metal oxide can be given. In addition, oxides of metalsbelonging to Group 4 to Group 8 of the periodic table can be given.Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromiumoxide, molybdenum oxide, tungsten oxide, manganese oxide, and rheniumoxide are preferable because of their high electron-acceptingproperties. Among these, molybdenum oxide is especially preferablebecause it is stable in the air, has a low hygroscopic property, and iseasily handled.

As the material with a hole-transport property used for the compositematerial, any of a variety of organic compounds such as aromatic aminecompounds, carbazole derivatives, aromatic hydrocarbons, and highmolecular compounds (e.g., oligomers, dendrimers, or polymers) can beused. Note that the organic compound used for the composite material ispreferably an organic compound having a high hole-transport property.Specifically, a substance having a hole mobility of 10⁻⁶ cm²/Vs orhigher is preferably used. Organic compounds which can be used as thematerial having a hole-transport property in the composite material arespecifically given below.

Examples of the aromatic amine compound includeN,N′-di(p-tolyl)-N,N′-diphenyl-p-phenylenediamine (abbreviation:DTDPPA), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl(abbreviation: DPAB),N,N′-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenye-N-phenylamino]benzene (abbreviation:DPA3B), and the like.

As carbazole derivatives which can be used for the composite material,the following can be given specifically:3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1);3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2);3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1); and the like.

In addition, examples of the carbazole derivatives which can be used forthe composite material include 4,4′-di(N-carbazolyl)biphenyl(abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene(abbreviation: TCPB), 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA),1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene, and thelike.

Examples of the aromatic hydrocarbon which can be used for the compositematerial include 2-tert-butyl-9,10-di(2-naphthyl)anthracene(abbreviation: t-BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),2-tert-butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene,tetracene, rubrene, perylene, and 2,5,8,11-tetra(tert-butyl)perylene.Besides, pentacene, coronene, or the like can also be used. The aromatichydrocarbon which has a hole mobility of 1×10⁻⁶ cm²/Vs or more and whichhas 14 to 42 carbon atoms is particularly preferable.

The aromatic hydrocarbon which can be used for the composite materialmay have a vinyl skeleton. Examples of the aromatic hydrocarbon having avinyl group include 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation:DPVBi), 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation:DPVPA), and the like.

Moreover, a high molecular compound such as poly(N-vinylcarbazole)(abbreviation: PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), orpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine (abbreviation:poly-TPD) can also be used. Note that as the material having ahole-transport property in the composite material, the organic compounddescribed in Embodiment 1 can also be used.

By providing a hole-injection layer, a high hole-injection property canbe achieved to allow a light-emitting element to be driven at a lowvoltage.

The hole-transport layer is a layer containing a material having ahole-transport property. Examples of the material having ahole-transport property include aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP), and the like. The substances given here havehigh hole-transport properties and are mainly ones having a holemobility of 10⁻⁶ cm²/Vs or more. An organic compound given as an exampleof the material with a hole-transport property in the composite materialdescribed above can also be used for the hole-transport layer. Moreover,a high molecular compound such as poly(N-vinylcarbazole) (abbreviation:PVK) or poly(4-vinyltriphenylamine) (abbreviation: PVTPA) can also beused. The organic compound described in Embodiment 1 can also be used.Note that the layer containing a material with a hole-transport propertyis not limited to a single layer, and may be a stack of two or morelayers containing any of the above substances.

Although the light-emitting layer 113, which has a function of emittinglight, may have any of a variety of structures and may include any of avariety of materials, it is preferable that the light-emitting layercontain the first organic compound with an electron-transport propertyand the second organic compound with a hole-transport property. Thelight-emitting layer may further contain a phosphorescent substance or afluorescent substance. Further preferable materials, structures, and thelike are as described in Embodiment 2 or 3. By having such a structure,the light-emitting element in this embodiment has extremely highexternal quantum efficiency. The light-emitting element also has anadvantage in that its emission wavelength can be easily adjusted andthus light in desired wavelength ranges can be easily obtained with theefficiency kept high.

The electron-transport layer 114 is a layer containing a material havingan electron-transport property. For example, the electron-transportlayer 114 is formed using a metal complex having a quinoline skeleton ora benzoquinoline skeleton, such as tris(8-quinolinolato)aluminum(abbreviation: Alq), tris(4-methyl-8-quinolinolato)aluminum(abbreviation: Almq₃), bis(10-hydroxybenzo[h]quinolinato)beryllium(abbreviation: BeBq₂), orbis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation:BAlq), or the like. A metal complex having an oxazole-based orthiazole-based ligand, such as bis[2-(2-hydroxyphenyl)benzoxazolato]zinc(abbreviation: Zn(BOX)₂) or bis[2-(2-hydroxyphenyl)benzothiazolato]zinc(abbreviation: Zn(BTZ)₂), or the like can also be used. Other than themetal complexes,2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), or the like can also be used. Thesubstances mentioned here have high electron-transport properties andare mainly ones that have an electron mobility of 10⁻⁶ cm²/Vs or more.Note that the above-described first organic compound with anelectron-transport property may be used for the electron-transport layer114.

The electron-transport layer 114 is not limited to a single layer, andmay be a stack including two or more layers containing any of the abovesubstances.

Between the electron-transport layer and the light-emitting layer, alayer that controls transport of electron carriers may be provided. Thisis a layer formed by addition of a small amount of a substance having ahigh electron-trapping property to the aforementioned materials having ahigh electron-transport property, and the layer is capable of adjustingcarrier balance by retarding transport of electron carriers. Such astructure is very effective in preventing a problem (such as a reductionin element lifetime) caused when electrons pass through thelight-emitting layer.

In addition, an electron-injection layer 115 may be provided in contactwith the second electrode 102 between the electron-transport layer 114and the second electrode 102. For the electron-injection layer 115, analkali metal, an alkaline earth metal, or a compound thereof, such aslithium fluoride (LiF), cesium fluoride (CsF), or calcium fluoride(CaF₂), can be used. For example, a layer that is formed using asubstance having an electron-transport property and contains an alkalimetal, an alkaline earth metal, or a compound thereof can be used. Notethat a layer that is formed using a substance having anelectron-transport property and contains an alkali metal or an alkalineearth metal is preferably used as the electron-injection layer 115, inwhich case electron injection from the second electrode 102 isefficiently performed.

For the second electrode 102, any of metals, alloys, electricallyconductive compounds, and mixtures thereof which have a low workfunction (specifically, a work function of 3.8 eV or less) or the likecan be used. Specific examples of such a cathode material are elementsbelonging to Groups 1 and 2 of the periodic table, such as alkali metals(e.g., lithium (Li) and cesium (Cs)), magnesium (Mg), calcium (Ca), andstrontium (Sr), alloys thereof (e.g., MgAg and AlLi), rare earth metalssuch as europium (Eu) and ytterbium (Yb), alloys thereof, and the like.However, when the electron-injection layer is provided between thesecond electrode 102 and the electron-transport layer, for the secondelectrode 102, any of a variety of conductive materials such as Al, Ag,ITO, or indium oxide-tin oxide containing silicon or silicon oxide canbe used regardless of the work function. Films of these electricallyconductive materials can be formed by a sputtering method, an inkjetmethod, a spin coating method, or the like.

Any of a variety of methods can be used to form the layer 103 containingan organic compound regardless whether it is a dry process or a wetprocess. For example, a vacuum evaporation method, an inkjet method, aspin coating method, or the like may be used. Different formationmethods may be used for the electrodes or the layers.

In addition, the electrode may be formed by a wet method using a sol-gelmethod, or by a wet method using paste of a metal material.Alternatively, the electrode may be formed by a dry method such as asputtering method or a vacuum evaporation method.

In the light-emitting element having the above-described structure,current flows due to a potential difference between the first electrode101 and the second electrode 102, and holes and electrons recombine inthe light-emitting layer 113 which contains a substance having a highlight-emitting property, so that light is emitted. In other words, alight-emitting region is formed in the light-emitting layer 113.

Light emission is extracted out through one or both of the firstelectrode 101 and the second electrode 102. Therefore, one or both ofthe first electrode 101 and the second electrode 102 arelight-transmitting electrodes. In the case where only the firstelectrode 101 is a light-transmitting electrode, light emission isextracted through the first electrode 101. In the case where only thesecond electrode 102 is a light-transmitting electrode, light emissionis extracted through the second electrode 102. In the case where boththe first electrode 101 and the second electrode 102 arelight-transmitting electrodes, light emission is extracted through thefirst electrode 101 and the second electrode 102.

The structure of the layers provided between the first electrode 101 andthe second electrode 102 is not limited to the above-describedstructure. Preferably, a light-emitting region where holes and electronsrecombine is positioned away from the first electrode 101 and the secondelectrode 102 so that quenching due to the proximity of thelight-emitting region and a metal used for electrodes andcarrier-injection layers can be prevented.

Further, to inhibit transfer of energy from an exciton generated in thelight-emitting layer, preferably, the hole-transport layer and theelectron-transport layer which are in contact with the light-emittinglayer 113, particularly a carrier-transport layer in contact with a sidecloser to the light-emitting region in the light-emitting layer 113, areformed using a substance having a wider band gap than the exciplexincluded in the light-emitting layer.

FIG. 1B shows a light-emitting element having different structure fromFIG. 1A. One embodiment of a light-emitting element in which a pluralityof light-emitting units are stacked (hereinafter, also referred to as astacked-layer element) will be described with reference to FIG. 1B. Thislight-emitting element is a light-emitting element including a pluralityof light-emitting units between a first electrode and a secondelectrode. One light-emitting unit has a structure similar to that ofthe layer 103 containing an organic compound, which is illustrated inFIG. 1A. In other words, the light-emitting element illustrated in FIG.1A includes a single light-emitting unit; the light-emitting elementillustrated in FIG. 1B includes a plurality of light-emitting units.

In FIG. 1B, a first light-emitting unit 511 and a second light-emittingunit 512 are stacked between a first electrode 501 and a secondelectrode 502, and a charge-generation layer 513 is provided between thefirst light-emitting unit 511 and the second light-emitting unit 512.The first electrode 501 and the second electrode 502 correspond,respectively, to the first electrode 101 and the second electrode 102illustrated in FIG. 1A, and the materials given in the description forFIG. 1A can be used. Further, the first light-emitting unit 511 and thesecond light-emitting unit 512 may have the same structure or differentstructures.

The charge-generation layer 513 includes a composite material of anorganic compound and a metal oxide. As this composite material of anorganic compound and a metal oxide, the composite material that can beused for the hole-injection layer and shown in FIG. 1A can be used. Asthe organic compound, a variety of compounds such as an aromatic aminecompound, a carbazole compound, an aromatic hydrocarbon, and a highmolecular compound (such as an oligomer, a dendrimer, or a polymer) canbe used. An organic compound having a hole mobility of 1×10⁻⁶ cm²/Vs orhigher is preferably used. Note that any other substance may be used aslong as the substance has a hole-transport property higher than anelectron-transport property. The composite material of the organiccompound and the metal oxide can achieve low-voltage driving andlow-current driving because of the superior carrier-injection propertyand carrier-transport property. Note that in the light-emitting unitwhose anode side surface is in contact with the charge-generation layer,a hole-transport layer is not necessarily provided because thecharge-generation layer can also function as the hole-transport layer.

The charge-generation layer 513 may have a stacked-layer structure of alayer containing the composite material of an organic compound and ametal oxide and a layer containing another material. For example, astacked-layer structure of a layer containing the composite material ofan organic compound and a metal oxide and a layer containing a compoundselected from electron-donating substances and a compound having a highelectron-transport property may be formed. Moreover, a layer containingthe composite material of an organic compound and a metal oxide may bestacked with a transparent conductive film.

The charge-generation layer 513 interposed between the firstlight-emitting unit 511 and the second light-emitting unit 512 may haveany structure as long as electrons can be injected to a light-emittingunit on one side and holes can be injected to a light-emitting unit onthe other side when a voltage is applied between the first electrode 501and the second electrode 502. For example, in FIG. 1B, any layer can beused as the charge-generation layer 513 as long as the layer injectselectrons into the first light-emitting unit 511 and holes into thesecond light-emitting unit 512 when a voltage is applied such that thepotential of the first electrode is higher than that of the secondelectrode.

In FIG. 1B, the light-emitting element having two light-emitting unitsis described; however, one embodiment of the present invention can besimilarly applied to a light-emitting element in which three or morelight-emitting units are stacked. With a plurality of light-emittingunits partitioned by the charge-generation layer between a pair ofelectrodes as in the light-emitting element illustrated in FIG. 1B, itis possible to provide a light-emitting element which can emit lightwith high luminance with the current density kept low and has a longlifetime. In addition, a low-power-consumption light-emitting devicewhich can be driven at low voltage can be achieved.

The light-emitting units emit light having different colors from eachother, thereby obtaining light emission of a desired color in the wholelight-emitting element. For example, in a light-emitting element havingtwo light-emitting units, the first light-emitting unit gives red andgreen emissions and the second light-emitting unit gives blue emission,so that the light-emitting element can emit white light as the wholeelement.

The above-described structure can be combined with any of the structuresin this embodiment and the other embodiments.

A light-emitting element in this embodiment is preferably fabricatedover a substrate of glass, plastic, or the like. As the way of stackinglayers over the substrate, layers may be sequentially stacked from thefirst electrode 101 side or sequentially stacked from the secondelectrode 102 side. In a light-emitting device, although onelight-emitting element may be formed over one substrate, a plurality oflight-emitting elements may be formed over one substrate. With aplurality of light-emitting elements as described above formed over onesubstrate, a lighting device in which elements are separated or apassive-matrix light-emitting device can be manufactured. Alight-emitting element may be formed over an electrode electricallyconnected to a thin film transistor (TFT), for example, which is formedover a substrate of glass, plastic, or the like, so that an activematrix light-emitting device in which the TFT controls the drive of thelight-emitting element can be manufactured. Note that there is noparticular limitation on the structure of the TFT, which may be astaggered TFT or an inverted staggered TFT. In addition, crystallinityof a semiconductor used for the TFT is not particularly limited either;an amorphous semiconductor or a crystalline semiconductor may be used.In addition, a driver circuit formed in a TFT substrate may be formedwith an n-type TFT and a p-type TFT, or with either an n-type TFT or ap-type TFT.

Note that this embodiment can be combined with any of the otherembodiments as appropriate.

Embodiment 5

In this embodiment, a light-emitting device including the light-emittingelement described in any of Embodiments 2 to 4 is described.

In this embodiment, the light-emitting device manufactured using thelight-emitting element described in any of Embodiments 2 to 4 isdescribed with reference to FIGS. 2A and 2B. Note that FIG. 2A is a topview illustrating the light-emitting device and FIG. 2B is across-sectional view of FIG. 2A taken along lines A-B and C-D. Thislight-emitting device includes a driver circuit portion (source linedriver circuit) 601, a pixel portion 602, and a driver circuit portion(gate line driver circuit) 603, which control light emission of thelight-emitting element and denoted by dotted lines. Moreover, areference numeral 604 denotes a sealing substrate; 605, a sealingmaterial; and 607, a space surrounded by the sealing material 605.

Note that a lead wiring 608 is a wiring for transmitting signals to beinput to the source line driver circuit 601 and the gate line drivercircuit 603 and for receiving a video signal, a clock signal, a startsignal, a reset signal, and the like from an FPC (flexible printedcircuit) 609 serving as an external input terminal. Although only theFPC is illustrated here, a printed wiring board (PWB) may be attached tothe FPC. The light-emitting device in the present specificationincludes, in its category, not only the light-emitting device itself butalso the light-emitting device provided with the FPC or the PWB.

Next, a cross-sectional structure is described with reference to FIG.2B. The driver circuit portion and the pixel portion are formed over anelement substrate 610; the source line driver circuit 601, which is adriver circuit portion, and one of the pixels in the pixel portion 602are illustrated here.

In the source line driver circuit 601, a CMOS circuit is formed in whichan n-channel TFT 623 and a p-channel TFT 624 are combined. In addition,the driver circuit may be formed with any of a variety of circuits suchas a CMOS circuit, a PMOS circuit, or an NMOS circuit. Although adriver-integrated type in which the driver circuit is formed over thesubstrate is described in this embodiment, the present invention is notlimited to this type and the driver circuit can be formed outside thesubstrate.

The pixel portion 602 is formed with a plurality of pixels including aswitching TFT 611, a current controlling TFT 612, and a first electrode613 connected electrically with a drain of the current controlling TFT.An insulator 614 is formed to cover the end portions of the firstelectrode 613. Here, the insulator 614 is formed using a positive typephotosensitive acrylic resin film.

In order to improve the coverage, the insulator 614 is formed to have acurved surface with curvature at its upper or lower end portion. Forexample, in the case where positive photosensitive acrylic is used for amaterial of the insulator 614, only the upper end portion of theinsulator 614 preferably has a curved surface with a curvature radius(0.2 μm to 3 μm). As the insulator 614, either a negative photosensitiveresin or a positive photosensitive resin can be used.

A layer 616 containing an organic compound and a second electrode 617are fainted over the first electrode 613. As a material used for thefirst electrode 613 functioning as an anode, a material having a highwork function is preferably used. For example, a single-layer film of anITO film, an indium tin oxide film containing silicon, an indium oxidefilm containing zinc oxide at 2 wt % to 20 wt %, a titanium nitridefilm, a chromium film, a tungsten film, a Zn film, a Pt film, or thelike, a stack including a titanium nitride film and a film containingaluminum as its main component, a stack including three layers of atitanium nitride film, a film containing aluminum as its main component,and a titanium nitride film, or the like can be used. The stacked-layerstructure enables low wiring resistance, favorable ohmic contact, and afunction as an anode.

In addition, the layer 616 containing an organic compound is formed byany of a variety of methods such as an evaporation method using anevaporation mask, an inkjet method, and a spin coating method. The layer616 containing an organic compound has the structure described in any ofEmbodiments 2 to 4. Further, for another material included in the layer616 containing an organic compound, any of low molecular-weightcompounds and polymeric compounds (including oligomers and dendrimers)may be used.

As a material used for the second electrode 617, which is formed overthe layer 616 containing an organic compound and functions as a cathode,a material having a low work function (e.g., Al, Mg, Li, Ca, or an alloyor a compound thereof, such as MgAg, MgIn, or AlLi) is preferably used.In the case where light generated in the layer 616 containing an organiccompound is transmitted through the second electrode 617, a stackincluding a thin metal film and a transparent conductive film (e.g.,ITO, indium oxide containing zinc oxide at 2 wt % to 20 wt %, indium tinoxide containing silicon, or zinc oxide (ZnO)) is preferably used forthe second electrode 617.

Note that the light-emitting element is formed with the first electrode613, the layer 616 containing an organic compound, and the secondelectrode 617. The light-emitting element has the structure described inany of Embodiments 2 to 4. In the light-emitting device in thisembodiment, the pixel portion, which includes a plurality oflight-emitting elements, may include both the light-emitting elementdescribed in any of Embodiments 2 to 4 and a light-emitting elementhaving a different structure.

Further, the sealing substrate 604 is attached to the element substrate610 with the sealing material 605, so that the light-emitting element618 is provided in the space 607 surrounded by the element substrate610, the sealing substrate 604, and the sealing material 605. The space607 may be filled with filler, or may be filled with an inert gas (suchas nitrogen or argon), or the sealing material 605. It is preferablethat the sealing substrate be provided with a recessed portion and thedesiccant 625 be provided in the recessed portion, in which casedeterioration due to influence of moisture can be inhibited.

An epoxy-based resin or glass frit is preferably used for the sealingmaterial 605. It is preferable that such a material do not transmitmoisture or oxygen as much as possible. As the sealing substrate 604, aglass substrate, a quartz substrate, or a plastic substrate formed offiber reinforced plastic (FRP), polyvinyl fluoride) (PVF), polyester,acrylic, or the like can be used.

As described above, the light-emitting device which uses thelight-emitting element described in any of Embodiments 2 to 4 can beobtained.

The light-emitting device in this embodiment is fabricated using thelight-emitting element described in any of Embodiments 2 to 4 and thuscan have favorable characteristics. Specifically, since thelight-emitting element described in any of Embodiments 2 to 4 hasfavorable emission efficiency, the light-emitting device can havereduced power consumption. In addition, light in desired wavelengthranges can be easily provided by the light-emitting element described inany of Embodiments 2 to 4, which makes it possible to provide aversatile light-emitting device.

FIGS. 3A and 3B each illustrate an example of a light-emitting device inwhich full color display is achieved by formation of a light-emittingelement exhibiting white light emission and with the use of coloringlayers (color filters) and the like. In FIG. 3A, a substrate 1001, abase insulating film 1002, a gate insulating film 1003, gate electrodes1006, 1007, and 1008, a first interlayer insulating film 1020, a secondinterlayer insulating film 1021, a peripheral portion 1042, a pixelportion 1040, a driver circuit portion 1041, first electrodes 1024W,1024R, 1024G, and 1024B of light-emitting elements, a partition 1025, alayer 1028 containing an organic compound, a second electrode 1029 ofthe light-emitting elements, a sealing substrate 1031, a sealingmaterial 1032, and the like are illustrated.

In FIG. 3A, coloring layers (a red coloring layer 10348, a greencoloring layer 1034G, and a blue coloring layer 1034B) are provided on atransparent base material 1033. A black layer (a black matrix) 1035 maybe additionally provided. The transparent base material 1033 providedwith the coloring layers and the black layer is positioned and fixed tothe substrate 1001. Note that the coloring layers and the black layerare covered with an overcoat layer 1036. In FIG. 3A, light emitted frompart of the light-emitting layer does not pass through the coloringlayers, while light emitted from the other part of the light-emittinglayer passes through the coloring layers. Since light which does notpass through the coloring layers is white and light which passes throughany one of the coloring layers is red, blue, or green, an image can bedisplayed using pixels of the four colors.

FIG. 3B illustrates an example in which the coloring layers (the redcoloring layer 1034R, the green coloring layer 1034G, and the bluecoloring layer 1034B) are provided between the gate insulating film 1003and the first interlayer insulating film 1020. As in the structure, thecoloring layers may be provided between the substrate 1001 and thesealing substrate 1031.

The above-described light-emitting device is a light-emitting devicehaving a structure in which light is extracted from the substrate 1001side where the TFTs are fanned (a bottom emission structure), but may bea light-emitting device having a structure in which light is extractedfrom the sealing substrate 1031 side (a top emission structure). FIG. 4is a cross-sectional view of a light-emitting device having a topemission structure. In this case, a substrate which does not transmitlight can be used as the substrate 1001. The process up to the step offorming a connection electrode which connects the TFT and the anode ofthe light-emitting element is performed in a manner similar to that ofthe light-emitting device having a bottom emission structure. Then, athird interlayer insulating film 1037 is formed to cover an electrode1022. This insulating film may have a planarization function. The thirdinterlayer insulating film 1037 can be formed using a material similarto that of the second interlayer insulating film, and can alternativelybe formed using any other known material.

The first electrodes 1024W, 1024R, 1024G, and 1024B of thelight-emitting elements each function as an anode here, but may functionas a cathode. Further, in the case of a light-emitting device having atop emission structure as illustrated in FIG. 4, the first electrodesare preferably reflective electrodes. The layer 1028 containing anorganic compound is formed to have a structure similar to the structureof the layer 103 containing an organic compound, which is described inany of Embodiments 2 to 4, with which white light emission can beobtained.

In the case of a top emission structure as illustrated in FIG. 4,sealing can be performed with the sealing substrate 1031 on which thecoloring layers (the red coloring layer 1034R, the green coloring layer1034G, and the blue coloring layer 1034B) are provided. The sealingsubstrate 1031 may be provided with the black layer (the black matrix)1035 which is positioned between pixels. The coloring layers (the redcoloring layer 1034R, the green coloring layer 1034G, and the bluecoloring layer 1034B) and the black layer (the black matrix) 1035 may becovered with the overcoat layer 1036. Note that a light-transmittingsubstrate is used as the sealing substrate 1031.

Further, although an example in which full color display is performedusing four colors of red, green, blue, and white is shown here, there isno particular limitation and full color display using three colors ofred, green, and blue may be performed.

The light-emitting device in this embodiment is manufactured using thelight-emitting element described in any of Embodiments 2 to 4 and thuscan have favorable characteristics. Specifically, since thelight-emitting element described in any of Embodiments 2 to 4 hasfavorable emission efficiency, the light-emitting device can havereduced power consumption. In addition, light in desired wavelengthranges can be easily provided by the light-emitting element described inany of Embodiments 2 to 4, which makes it possible to provide aversatile light-emitting device.

An active matrix light-emitting device is described above, whereas apassive matrix light-emitting device is described below. FIGS. 5A and 5Billustrate a passive matrix light-emitting device manufactured using thepresent invention. FIG. 5A is a perspective view of the light-emittingdevice, and FIG. 5B is a cross-sectional view of FIG. 5A taken alongline X-Y. In FIGS. 5A and 5B, a layer 955 containing an organic compoundis provided between an electrode 952 and an electrode 956 over asubstrate 951. An end portion of the electrode 952 is covered with aninsulating layer 953. A partition layer 954 is provided over theinsulating layer 953. The sidewalls of the partition layer 954 areaslope such that the distance between both sidewalls is graduallynarrowed toward the surface of the substrate. In other words, a crosssection taken along the direction of the short side of the partitionwall layer 954 is trapezoidal, and the lower side (a side which is inthe same direction as a plane direction of the insulating layer 953 andin contact with the insulating layer 953) is shorter than the upper side(a side which is in the same direction as the plane direction of theinsulating layer 953 and not in contact with the insulating layer 953.The partition layer 954 thus provided can prevent defects in thelight-emitting element due to static electricity or the like. Further,the passive matrix light-emitting device can also have lower powerconsumption by including the light-emitting element described in any ofEmbodiments 2 to 4, which has favorable emission efficiency. Inaddition, light in desired wavelength ranges can be easily provided bythe light-emitting element described in any of Embodiments 2 to 4, whichmakes it possible to provide a versatile light-emitting device.

Since many minute light-emitting elements arranged in a matrix in thelight-emitting device described above can each be controlled, thelight-emitting device can be suitably used as a display device fordisplaying images.

This embodiment can be freely combined with any of other embodiments.

Embodiment 6

In this embodiment, an example in which the light-emitting elementdescribed in any of Embodiments 2 to 4 is used for a lighting device isdescribed with reference to FIGS. 6A and 6B. FIG. 6B is a top view ofthe lighting device, and FIG. 6A is a cross-sectional view of FIG. 6Btaken along line e-f.

In the lighting device in this embodiment, a first electrode 401 isformed over a substrate 400 which is a support and has alight-transmitting property. The first electrode 401 corresponds to thefirst electrode 101 in Embodiment 4. When light is extracted through thefirst electrode 401 side, the first electrode 401 is formed using amaterial having a light-transmitting property.

A pad 412 for applying voltage to a second electrode 404 is providedover the substrate 400.

A layer 403 containing an organic compound is formed over the firstelectrode 401. The structure of the layer 403 containing an organiccompound corresponds to, for example, the structure of the layer 103containing an organic compound in Embodiment 4, or the structure inwhich the light-emitting units 511 and 512 and the charge-generationlayer 513 are combined. For these structures, the description inEmbodiment 4 can be referred to.

The second electrode 404 is formed to cover the layer 403 containing anorganic compound. The second electrode 404 corresponds to the secondelectrode 102 in Embodiment 4. The second electrode 404 is formed usinga material having high reflectance when light is extracted through thefirst electrode 401 side. The second electrode 404 is connected to thepad 412, whereby voltage is applied thereto.

As described above, the lighting device described in this embodimentincludes a light-emitting element including the first electrode 401, thelayer 403 containing an organic compound, and the second electrode 404.Since the light-emitting element is inexpensive and excellent indurability, the lighting device in this embodiment can have highemission efficiency.

The light-emitting element having the above structure is fixed to asealing substrate 407 with a sealing material 405 and sealing isperformed, whereby the lighting device is completed. In addition, thesealing material 405 can be mixed with a desiccant which allows moistureto be adsorbed, increasing reliability.

When parts of the pad 412 and the first electrode 401 are extended tothe outside of the sealing material 405, the extended parts can serve asexternal input terminals. An IC chip 420 mounted with a converter or thelike may be provided over the external input terminals.

As described above, since the lighting device described in thisembodiment includes the light-emitting element described in any ofEmbodiments 2 to 4, the lighting device can have high emissionefficiency.

Embodiment 7

In this embodiment, examples of electronic devices each including thelight-emitting element described in any of Embodiments 2 to 4 aredescribed. The light-emitting element described in any of Embodiments 2to 4 has high emission efficiency and accordingly, the electronicdevices in this embodiment each of which includes the light-emittingelement can have low power consumption.

Examples of the electronic device to which the above light-emittingelement is applied include television devices (also referred to as TV ortelevision receivers), monitors for computers and the like, cameras suchas digital cameras and digital video cameras, digital photo frames,mobile phones (also referred to as cell phones or mobile phone devices),portable game machines, portable information terminals, audio playbackdevices, large game machines such as pachinko machines, and the like.Specific examples of these electronic devices are described below.

FIG. 7A illustrates an example of a television device. In the televisiondevice, a display portion 7103 is incorporated in a housing 7101. Here,the housing 7101 is supported by a stand 7105. Images can be displayedon the display portion 7103, and in the display portion 7103, thelight-emitting elements described in any of Embodiments 2 to 4 arearranged in a matrix. The light-emitting elements can have high emissionefficiency. Therefore, the television device including the displayportion 7103 which is formed using the light-emitting element can havelow power consumption.

The television device can be operated with an operation switch of thehousing 7101 or a separate remote controller 7110. With operation keys7109 of the remote controller 7110, channels and volume can becontrolled and images displayed on the display portion 7103 can becontrolled. Further, the remote controller 7110 may be provided with adisplay portion 7107 for displaying data output from the remotecontroller 7110.

Note that the television device is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the television device isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

FIG. 7B1 illustrates a computer, which includes a main body 7201, ahousing 7202, a display portion 7203, a keyboard 7204, an externalconnection port 7205, a pointing device 7206, and the like. Note thatthis computer is manufactured using light-emitting elements arranged ina matrix in the display portion 7203, which are the same as thatdescribed in any of Embodiments 2 to 4. The computer illustrated in FIG.7B1 may have a structure illustrated in FIG. 7B2. The computerillustrated in FIG. 7B2 is provided with a second display portion 7210instead of the keyboard 7204 and the pointing device 7206. The seconddisplay portion 7210 is a touchscreen, and input can be performed byoperation of display for input on the second display portion 7210 with afinger or a dedicated pen. The second display portion 7210 can alsodisplay images other than the display for input. The display portion7203 may also be a touchscreen. Connecting the two screens with a hingecan prevent troubles; for example, the screens can be prevented frombeing cracked or broken while the computer is being stored or carried.Note that this computer is manufactured using light-emitting elementsarranged in a matrix in the display portion 7203, which are the same asthat described in any of Embodiments 2 to 4. Therefore, this computerhaving the display portion 7203 which is formed using the light-emittingelements consumes less power.

FIG. 7C illustrates a portable game machine, which includes twohousings, a housing 7301 and a housing 7302, which are connected with ajoint portion 7303 so that the portable game machine can be opened orfolded. The housing 7301 incorporates a display portion 7304 includingthe light-emitting elements each of which is described in any ofEmbodiments 2 to 4 and which are arranged in a matrix, and the housing7302 incorporates a display portion 7305. In addition, the portable gamemachine illustrated in FIG. 7C includes a speaker portion 7306, arecording medium insertion portion 7307, an LED lamp 7308, an input unit(an operation key 7309, a connection terminal 7310, a sensor 7311 (asensor having a function of measuring force, displacement, position,speed, acceleration, angular velocity, rotational frequency, distance,light, liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radiation,flow rate, humidity, gradient, oscillation, odor, or infrared rays), anda microphone 7312), and the like. Needless to say, the structure of theportable game machine is not limited to the above as long as the displayportion including the light-emitting elements each of which is describedin any of Embodiments 2 to 4 and which are arranged in a matrix is usedas at least either the display portion 7304 or the display portion 7305,or both, and the structure can include other accessories as appropriate.The portable game machine illustrated in FIG. 7C has a function ofreading out a program or data stored in a storage medium to display iton the display portion, and a function of sharing information withanother portable game machine by wireless communication. The portablegame machine illustrated in FIG. 7C can have a variety of functionswithout limitation to the above. The portable game machine having thedisplay portion 7304 can have low power consumption because thelight-emitting element described in any of Embodiments 2 to 4 is used inthe display portion 7304.

FIG. 7D illustrates an example of a mobile phone. The mobile phone isprovided with a display portion 7402 incorporated in a housing 7401,operation buttons 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone hasthe display portion 7402 including the light-emitting elements each ofwhich is described in any of Embodiments 2 to 4 and which are arrangedin a matrix. Accordingly, the mobile phone can have low powerconsumption.

When the display portion 7402 of the mobile phone illustrated in FIG. 7Dis touched with a finger or the like, data can be input into the mobilephone. In this case, operations such as making a call and creatinge-mail can be performed by touch on the display portion 7402 with afinger or the like.

There are mainly three screen modes of the display portion 7402. Thefirst mode is a display mode mainly for displaying images. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is a display-and-input mode in which two modes of the display modeand the input mode are combined.

For example, in the case of making a call or composing e-mail, a textinput mode mainly for inputting text is selected for the display portion7402 so that text displayed on a screen can be inputted. In this case,it is preferable to display a keyboard or number buttons on almost theentire screen of the display portion 7402.

When a detection device which includes a sensor for detectinginclination, such as a gyroscope or an acceleration sensor, is providedinside the mobile phone, the direction of the mobile phone (whether themobile phone is placed horizontally or vertically for a landscape modeor a portrait mode) is determined so that display on the screen of thedisplay portion 7402 can be automatically switched.

The screen modes are switched by touching the display portion 7402 oroperating the operation buttons 7403 of the housing 7401. Alternatively,the screen modes can be switched depending on the kinds of imagesdisplayed on the display portion 7402. For example, when a signal of animage displayed on the display portion is a signal of moving image data,the screen mode is switched to the display mode. When the signal is asignal of text data, the screen mode is switched to the input mode.

Moreover, in the input mode, when input by touching the display portion7402 is not performed within a specified period while a signal detectedby an optical sensor in the display portion 7402 is detected, the screenmode may be controlled so as to be switched from the input mode to thedisplay mode.

The display portion 7402 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken by touchon the display portion 7402 with the palm or the finger, wherebypersonal authentication can be performed. Further, by providing abacklight or a sensing light source which emits a near-infrared light inthe display portion, an image of a finger vein, a palm vein, or the likecan be taken.

FIG. 8 illustrates an example of a liquid crystal display device usingthe light-emitting element described in any of Embodiments 2 to 4 for abacklight. The liquid crystal display device shown in FIG. 8 includes ahousing 901, a liquid crystal layer 902, a backlight unit 903, and ahousing 904. The liquid crystal layer 902 is connected to a driver IC905. The light-emitting element described in any of Embodiments 2 to 4is used for the backlight unit 903, to which current is supplied througha terminal 906.

The light-emitting element described in any of Embodiments 2 to 4 isused for the backlight of the liquid crystal display device; thus, thebacklight can have reduced power consumption. In addition, the use ofthe light-emitting element described in any of Embodiments 2 to 4enables manufacture of a planar-emission lighting device and further alarger-area planar-emission lighting device; therefore, the backlightcan be a larger-area backlight, and the liquid crystal display devicecan also be a larger-area device. Furthermore, the light-emitting deviceusing the light-emitting element described in any of Embodiments 2 to 4can be thinner than a conventional one; accordingly, the display devicecan also be thinner.

FIG. 9 illustrates an example in which the light-emitting elementdescribed in any of Embodiments 2 to 4 is used for a table lamp which isa lighting device. The table lamp illustrated in FIG. 9 includes ahousing 2001 and a light source 2002, and the lighting device describedin Embodiment 6 is used for the light source 2002.

FIG. 10 illustrates an example in which the light-emitting elementdescribed in any of Embodiments 2 to 4 is used for an indoor lightingdevice 3001. Since the light-emitting element described in any ofEmbodiments 2 to 4 has low power consumption, a lighting device havinglow power consumption can be obtained. Further, since the light-emittingelement described in any of Embodiments 2 to 4 can have a large area,the light-emitting element can be used for a large-area lighting device.Furthermore, since the light-emitting element described in any ofEmbodiments 2 to 4 is thin, the light-emitting element can be used for alighting device having a reduced thickness.

The light-emitting element described in any of Embodiments 2 to 4 canalso be used for an automobile windshield or an automobile dashboard.FIG. 11 illustrates one mode in which the light-emitting elementdescribed in any of Embodiments 2 to 4 is used for an automobilewindshield and an automobile dashboard. Display regions 5000 to 5005each include the light-emitting element described in any of Embodiments2 to 4.

The display regions 5000 and the display region 5001 display deviceswhich are provided in the automobile windshield and in which thelight-emitting elements described in any of Embodiments 2 to 4 areincorporated. The light-emitting element described in any of Embodiments2 to 4 can be formed into what is called a see-through display device,through which the opposite side can be seen, by including a firstelectrode and a second electrode formed of electrodes havinglight-transmitting properties. Such see-through display devices can beprovided even in the automobile windshield, without hindering thevision. Note that in the case where a transistor for driving or the likeis provided, a transistor having a light-transmitting property, such asan organic transistor using an organic semiconductor material or atransistor using an oxide semiconductor, is preferably used.

A display device incorporating the light-emitting element described inany of Embodiments 2 to 4 is provided in the display region 5002 in apillar portion. The display region 5002 can compensate for the viewhindered by the pillar portion by showing an image taken by an imagingunit provided in the car body. Similarly, the display region 5003provided in the dashboard can compensate for the view hindered by thecar body by showing an image taken by an imaging unit provided in theoutside of the car body, which leads to elimination of blind areas andenhancement of safety. Showing an image so as to compensate for the areawhich a driver cannot see makes it possible for the driver to confirmsafety easily and comfortably.

The display region 5004 and the display region 5005 can provide avariety of kinds of information such as navigation data, a speedometer,a tachometer, a mileage, a fuel meter, a gearshift indicator, andair-condition setting. The content or layout of the display can bechanged freely by a user as appropriate. Further, such information canalso be shown by the display regions 5000 to 5003. Note that the displayregions 5000 to 5005 can also be used as lighting devices.

The light-emitting element described in any of Embodiments 2 to 4 canhave low power consumption.

For that reason, load on a battery is small even when a number of largescreens such as the display regions 5000 to 5005 are provided, whichprovides comfortable use. For that reason, the light-emitting device andthe lighting device each of which includes the light-emitting elementdescribed in any of Embodiments 2 to 4 can be suitably used as anin-vehicle light-emitting device and an in-vehicle lighting device.

FIGS. 12A and 12B illustrate an example of a foldable tablet terminal.The tablet terminal is opened in FIG. 12A. The tablet terminal includesa housing 9630, a display portion 9631 a, a display portion 9631 b, adisplay mode switch 9034, a power switch 9035, a power saver switch9036, a clasp 9033, and an operation switch 9038. Note that in thetablet terminal, one or both of the display portion 9631 a and thedisplay portion 9631 b is/are formed using a light-emitting device whichincludes the light-emitting element described in any of Embodiments 2 to4.

Part of the display portion 9631 a can be a touchscreen region 9632 aand data can be input when a displayed operation key 9637 is touched.Although half of the display portion 9631 a has only a display functionand the other half has a touchscreen function, one embodiment of thepresent invention is not limited to the structure. The whole displayportion 9631 a may have a touchscreen function. For example, a keyboardcan be displayed on the entire region of the display portion 9631 a sothat the display portion 9631 a is used as a touchscreen, and thedisplay portion 9631 b can be used as a display screen.

Like the display portion 9631 a, part of the display portion 9631 b canbe a touchscreen region 9632 b. A switching button 9639 forshowing/hiding a keyboard of the touchscreen is touched with a finger, astylus, or the like, so that keyboard buttons can be displayed on thedisplay portion 9631 b.

Touch input can be performed in the touchscreen region 9632 a and thetouchscreen region 9632 b at the same time.

The display mode switch 9034 can switch the display between portraitmode, landscape mode, and the like, and between monochrome display andcolor display, for example. With the power saver switch 9036, theluminance of display can be optimized in accordance with the amount ofexternal light at the time when the tablet terminal is in use, which isdetected with an optical sensor incorporated in the tablet terminal. Thetablet terminal may include another detection device such as a sensorfor detecting orientation (e.g., a gyroscope or an acceleration sensor)in addition to the optical sensor.

Although FIG. 12A illustrates an example in which the display portion9631 a and the display portion 9631 b have the same display area, oneembodiment of the present invention is not limited to the example. Thedisplay portion 9631 a and the display portion 9631 b may have differentdisplay areas and different display quality. For example, one of themmay be a display panel that can display higher-definition images thanthe other.

The tablet terminal is folded in FIG. 12B. The tablet terminal includesthe housing 9630, a solar cell 9633, a charge and discharge controlcircuit 9634, a battery 9635, and a DC-to-DC converter 9636. Note thatFIG. 12B illustrates an example in which the charge and dischargecontrol circuit 9634 includes the battery 9635 and the DC-to-DCconverter 9636.

Since the tablet terminal can be folded, the housing 9630 can be closedwhen not in use. Thus, the display portions 9631 a and 9631 b can beprotected, thereby providing a tablet terminal with high endurance andhigh reliability for long-term use.

In addition, the tablet terminal illustrated in FIGS. 12A and 12B canhave a function of displaying various kinds of information (e.g., astill image, a moving image, and a text image) on the display portion, afunction of displaying a calendar, the date, the time, or the like onthe display portion, a touch input function of operating or editinginformation displayed on the display portion by touch input, a functionof controlling processing by various kinds of software (programs), andthe like.

The solar cell 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touchscreen, a display portion,an image signal processor, and the like. Note that the solar cell 9633is preferably provided on one or two surfaces of the housing 9630, inwhich case the battery 9635 can be charged efficiently.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 12B are described with reference to a blockdiagram of FIG. 12C. FIG. 12C shows the solar cell 9633, the battery9635, the DC-to-DC converter 9636, a converter 9638, switches SW1 toSW3, and the display portion 9631. The battery 9635, the DC-to-DCconverter 9636, the converter 9638, and the switches SW1 to SW3correspond to the charge and discharge control circuit 9634 in FIG. 12B.

First, an example of operation in the case where power is generated bythe solar cell 9633 using external light is described. The voltage ofpower generated by the solar cell is raised or lowered by the DC-to-DCconverter 9636 so that the power has voltage for charging the battery9635. Then, when power supplied from the battery 9635 charged by thesolar cell 9633 is used for the operation of the display portion 9631,the switch SW1 is turned on and the voltage of the power is raised orlowered by the converter 9638 so as to be voltage needed for the displayportion 9631. In addition, when display on the display portion 9631 isnot performed, the switch SW1 is turned off and a switch SW2 is turnedon so that charge of the battery 9635 may be performed.

Although the solar cell 9633 is described as an example of a powergeneration unit, the power generation unit is not particularly limited,and the battery 9635 may be charged by another power generation unitsuch as a piezoelectric element or a thermoelectric conversion element(Peltier element). The battery 9635 may be charged by a non-contactpower transmission module which is capable of charging by transmittingand receiving power by wireless (without contact), or any of the othercharge unit used in combination, and the power generation unit is notnecessarily provided.

One embodiment of the present invention is not limited to the tabletterminal having the shape illustrated in FIGS. 12A to 12C as long as thedisplay portion 9631 is included.

Note that the structure described in this embodiment can be combinedwith any of the structures described in Embodiments 1 to 6 asappropriate:

As described above, the application range of the light-emitting elementdescribed in any of Embodiments 2 to 4 is so wide that thislight-emitting element can be applied to electronic devices in a varietyof fields. By using the light-emitting element described in any ofEmbodiments 2 to 4, an electronic device having low power consumptioncan be obtained.

Example 1

In this example, a method for synthesizingN-(1,1′-biphenyl-4-yl)-N-[4-(dibenzofuran-4-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: FrBBiF-II) represented by Structural Formula (100) willbe described.

First, 2.1 g (6.6 mmol) of 4-(4-bromophenyl)dibenzofuran, 2.4 g (6.7mmol) of N-(1,1′-biphenyl-4-yl)-9,9-dimethyl-9H-fluoren-2-amine, and 1.9g (20 mmol) of sodium tert-butoxide were put in a 200-mL three-neckflask and the air in the flask was replaced with nitrogen. To thismixture, 33 mL of toluene, 0.30 mL of a 10% hexane solution oftri(tert-butyl)phosphine, and 48 mg (0.1 mmol) ofbis(dibenzylideneacetone)palladium(0) were added, and stirring wasperformed at 90° C. for 7.5 hours. After the stirring, suctionfiltration through Florisil (produced by Wako Pure Chemical Industries,Ltd., Catalog No. 540-00135), Celite (produced by Wako Pure ChemicalIndustries, Ltd., Catalog No. 531-16855), and alumina was carried out togive a filtrate. The filtrate was concentrated to give a solid. Thesolid was purified by silica gel column chromatography (the developingsolvent was hexane and toluene in a ratio of 3:1) to give a solid. Thesolid was recrystallized from toluene and hexane, so that 3.2 g of anobjective solid was obtained in a yield of 81%. A reaction scheme ofthis reaction is shown below.

Using a train sublimation method, 1.0 g of the obtained solid waspurified by sublimation. In the purification by sublimation, thepressure was 2.6 Pa, the flow rate of argon gas was 5.0 mL/min, and thetemperature of the heating was 289° C. After the purification bysublimation, 0.99 g of a solid which was the object of the synthesis wasobtained at a collection rate of 95%.

Results of measurement of the obtained solid by nuclear magneticresonance (¹H NMR) are shown below.

¹H NMR (CDCl₃, 500 MHz): δ=1.46 (s, 6H), 7.18 (dd, J=8.5 Hz, 2.5 Hz,1H), 7.26-7.48 (m, 14H), 7.53-7.56 (m, 2H), 7.60-7.68 (m, 6H), 7.86-7.91(m, 3H), 7.99 (d, J=7.5 Hz, 1H).

Thermogravimetry-differential thermal analysis (TG-DTA) of the obtainedFrBBiF-II was performed. The measurement was conducted by using a highvacuum differential type differential thermal balance (TG/DTA 2410SA,manufactured by Bruker AXS K.K.). The measurement was carried out undera nitrogen stream (a flow rate of 200 mL/min) and a normal pressure at atemperature rising rate of 10° C./min. The relationship between weightand temperature (thermogravimetry) shows that the 5% weight losstemperature is 393° C., which is indicative of high heat resistance.

FIGS. 13A and 13B are NMR charts. Note that FIG. 13B shows an enlargedpart of FIG. 13A in the range of 7.00 ppm to 8.25 ppm. The measurementresults confirmed thatN-(1,1′-biphenyl-4-yl)-N-[4-(dibenzofuran-4-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: FrBBiF-II), which was the target substance, was obtained.

<<Physical Properties of FrBBiF-II>>

FIG. 14A shows an absorption spectrum and an emission spectrum ofFrBBiF-II in a toluene solution of FrBBiF-II, and FIG. 14B shows anabsorption spectrum and an emission spectrum of a thin film ofFrBBiF-II. The spectra were measured with a UV-visible spectrophotometer(V550, produced by JASCO Corporation). The spectra of FrBBiF-II in thetoluene solution of FrBBiF-II were measured with a toluene solution ofFrBBiF-II put in a quartz cell. The spectra of the thin film weremeasured with a sample prepared by deposition of FrBBiF-II on a quartzsubstrate by evaporation. Note that in the case of the absorptionspectrum of FrBBiF-II in the toluene solution of FrBBiF-II, theabsorption spectrum obtained by subtraction of the absorption spectra ofthe quartz cell and toluene from the measured spectra is shown in thedrawing and that in the case of the absorption spectrum of the thin filmof FrBBiF-II, the absorption spectrum obtained by subtraction of theabsorption spectrum of the quartz substrate from the measured spectra isshown in the drawing.

As shown in FIG. 14A, in the case of FrBBiF-II in the toluene solution,an absorption peak was observed at approximately 360 nm, and an emissionwavelength peak was observed at approximately 415 nm (excitationwavelength: 366 nm). As shown in FIG. 14B, in the case of the thin filmof FrBBiF-II, absorption peaks were observed at approximately 368 nm,294 nm, 266 nm, 247 nm, and 209 nm, and an emission wavelength peak wasobserved at approximately 428 nm (excitation wavelength: 376 nm). Thus,it was found that absorption and light emission of FrBBiF-II occur inextremely short wavelength regions.

The ionization potential of FrBBiF-II in a thin film state was measuredby photoelectron spectroscopy (the measuring instrument: AC-2,manufactured by Riken Keiki, Co., Ltd.) in the air. The obtained valueof the ionization potential was converted into a negative value, so thatthe HOMO level of FrBBiF-II was determined to be −5.61 eV. From the dataof the absorption spectrum of the thin film in FIG. 14B, the absorptionedge of FrBBiF-II, which was obtained from Tauc plot with an assumptionof direct transition, was 3.11 eV. Therefore, the optical energy gap ofFrBBiF-II in a solid state was estimated at 3.11 eV; from the values ofthe HOMO level obtained above and this energy gap, the LUMO level ofFrBBiF-II was estimated at −2.50 eV. This reveals that FrBBiF-II in thesolid state has an energy gap as wide as 3.11 eV.

Furthermore, FrBBiF-II was analyzed by liquid chromatography massspectrometry (LC/MS).

The analysis by LC/MS was carried out with Acquity UPLC (produced byWaters Corporation) and Xevo G2 T of MS (produced by WatersCorporation).

In the MS analysis, ionization was carried out by an electrosprayionization (ESI) method. Capillary voltage and sample cone voltage wereset to 3.0 kV and 30 V, respectively. Detection was performed in apositive mode.

A component which underwent the ionization under the above-mentionedconditions was collided with an argon gas in a collision cell todissociate into product ions. Energy (collision energy) for thecollision with argon was 50 eV. A mass range for the measurement wasm/z=100 to 1200. FIG. 15 shows the results.

Example 2

In this example, the light-emitting element (a light-emitting element 1)described in Embodiment 2 will be described. Note that in thelight-emitting element 1,N-(1,1′-biphenyl-4-yl)-N-[4-(dibenzofuran-4-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: FrBBiF-II), which is the organic compound described inEmbodiment 1, was used as the second organic compound in thelight-emitting layer 113, and4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation: 4,6mCzP2Pm)was used as the first organic compound. Chemical formulae of materialsused in this example are shown below.

(Method for Fabricating Light-Emitting Element 1)

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate by a sputtering method, so that the firstelectrode 101 was formed. The thickness thereof was 110 nm and theelectrode area was 2 mm×2 mm. Here, the first electrode 101 is anelectrode that functions as an anode of the light-emitting element.

Next, as pretreatment for forming the light-emitting element over thesubstrate, UV ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for 1 hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for approximately 30 minutes.

Then, the substrate over which the first electrode 101 was formed wasfixed to a substrate holder provided in the vacuum evaporation apparatusso that the surface on which the first electrode 101 was formed faceddownward. The pressure in the vacuum evaporation apparatus was reducedto approximately 10⁻⁴ Pa. After that, over the first electrode 101,4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II) represented by Structural Formula (i) and molybdenum(VI) oxidewere deposited by co-evaporation by an evaporation method usingresistance heating, so that the hole-injection layer 111 was formed. Thethickness of the hole-injection layer 111 was set to 20 nm, and theweight ratio of DBT3P-II to molybdenum oxide was adjusted to 4:2. Notethat the co-evaporation method refers to an evaporation method in whichevaporation is carried out from a plurality of evaporation sources atthe same time in one treatment chamber.

Next, a film of 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP) which is represented by Structural Formula (ii)was formed to a thickness of 20 nm over the hole-injection layer 111 toform the hole-transport layer 112.

Further, over the hole-transport layer 112,4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation: 4,6mCzP2Pm)represented by Structural Formula (iii) andN-(1,1′-biphenyl-4-yl)-N-[4-(dibenzofuran-4-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: FrBBiF-II) represented by Structural Formula (100) weredeposited by co-evaporation to a thickness of 40 nm such that the weightratio of 4,6mCzP2Pm to FrBBiF-II was 0.8:0.2, whereby the light-emittinglayer 113 was formed.

Then, the electron-transport layer 114 was formed over thelight-emitting layer 113 in such a way that a 15 nm thick film of4,6mCzP2Pm was formed and a 15 nm thick film of bathophenanthroline(abbreviation: BPhen) represented by Structural Formula (iv) was formed.

After the formation of the electron-transport layer 114, lithiumfluoride (LiF) was deposited by evaporation to a thickness of 1 nm, sothat the electron-injection layer 115 was formed. Lastly, aluminum wasdeposited by evaporation to a thickness of 200 nm to form the secondelectrode 102 functioning as a cathode. Thus, the light-emitting element1 in this example was fabricated.

Note that in all the above evaporation steps, evaporation was performedby a resistance-heating method.

(Method for Fabricating Comparative Light-Emitting Element 1)

A comparative light-emitting element 1 was fabricated in a mannersimilar to that of the light-emitting element 1 except that FrBBiF-IIwas replaced withbis(biphenyl-4-yl)[4′-(9-phenyl-9H-carbazol-3-yl)biphenyl-4-yl]amine(abbreviation: PCTBi1BP) represented by Structural Formula (v) and thethickness of the film of Bphen was set to 10 nm.

The light-emitting element 1 and the comparative light-emitting element1 were each sealed using a glass substrate in a glove box containing anitrogen atmosphere so as not to be exposed to the air (specifically, asealing material was applied onto an outer edge of the element and heattreatment was performed at 80° C. for 1 hour at the time of sealing).Then, initial characteristics of these light-emitting elements weremeasured. Note that the measurements were carried out at roomtemperature (in an atmosphere kept at 25° C.).

FIG. 16 shows current density-luminance characteristics of thelight-emitting element 1 and the comparative light-emitting element 1;FIG. 17 shows luminance-current efficiency characteristics thereof; FIG.18 shows voltage-luminance characteristics thereof; FIG. 19 showsluminance-power efficiency characteristics thereof; FIG. 20 showsluminance-external quantum efficiency characteristics thereof; and FIG.21 shows emission spectra thereof.

Table 1 shows values of major characteristics of the light-emittingelement 1 and the comparative light-emitting element 1 at approximately1000 cd/m².

TABLE 1 External Current Current Power Quantum Voltage Current DensityEfficiency Efficiency Efficiency (V) (mA) (mA/cm²) Chromaticity xChromaticity y (cd/A) (lm/W) (%) Light-emitting 3.9 0.73 18.2 0.25 0.495.5 4.4 1.9 Element 1 Comparative 4.4 0.93 23.2 0.23 0.45 3.8 2.7 1.4Light-emitting Element 1

The above results show that the light-emitting element 1 usingFrBBiF-II, which is the organic compound described in Embodiment 1, hasfavorable characteristics. Specifically, according to FIG. 20 showingthe external quantum efficiencies, the light-emitting element 1 has anefficiency far exceeding 5% in a low-luminance region. It is said thatthe extraction efficiency of a light-emitting element like thelight-emitting element in this example which is not designed to enhanceextraction efficiency is approximately 20% to 30%. The theoretical limitof the internal quantum efficiency of fluorescence, which is based onthe generation ratio of singlet excitons generated by currentexcitation, is 25%. Thus, the theoretical limit of the external quantumefficiency of a fluorescent light-emitting element is calculated to be5% to 7.5%. It can be found that the external quantum efficiency of thelight-emitting element 1 in a low-luminance region far exceeds thetheoretical limit.

The above results suggest that an exciplex formed by the first organiccompound and the second organic compound (4,6mCzP2Pm and FrBBiF-II)emits light with high efficiency and that the light includes delayedfluorescence components. The light-emitting element 1 was able to emitlight with high emission efficiency owing to delayed fluorescence thatoccurred efficiently via reverse intersystem crossing from a tripletexcited state to a singlet excited state.

It was also found that the light-emitting element 1 is a light-emittingelement with low drive voltage.

In contrast, the comparative light-emitting element 1 includingPCTBi1BP, whose structure is similar to that of FrBBiF-II, has lowefficiency. This is probably because the T₁ level of PCTBi1BP waslowered owing to its structure in which the biphenyldiyl group connectsthe 9-phenylcarbazol-3-yl group and the nitrogen atom of the amine,i.e., a structure including a terphenyl skeleton, in which the number ofphenyl groups is one more than the number of phenyl groups in a biphenylskeleton. It can be thus considered that the T₁ level of PCTBi1BP becamelower than that of the exciplex and the excitation energy of theexciplex was deactivated, leading to such an insufficient efficiency.

Example 3

In this example, the light-emitting element (a light-emitting element 2)described in Embodiment 3 will be described. Note that in thelight-emitting element 2,N-(1,1′-biphenyl-4-yl)-N-[4-(dibenzofuran-4-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: FrBBiF-II), which is the organic compound described inEmbodiment 1, was used as the second organic compound in thelight-emitting layer 113,2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II) was used as the first organic compound,andbis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III) (abbreviation: [Ir(tBuppm)₂(acac)]) was used as an emission centersubstance emitting phosphorescence. Chemical formulae of materials usedin this example are shown below.

(Method for Fabricating Light-Emitting Element 2)

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate by a sputtering method, so that the firstelectrode 101 was formed. The thickness thereof was 110 nm and theelectrode area was 2 mm×2 mm. Here, the first electrode 101 is anelectrode that functions as an anode of the light-emitting element.

Next, as pretreatment for forming the light-emitting element over thesubstrate, UV ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for 1 hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for approximately 30 minutes.

Then, the substrate over which the first electrode 101 was formed wasfixed to a substrate holder provided in the vacuum evaporation apparatusso that the surface on which the first electrode 101 was formed faceddownward. The pressure in the vacuum evaporation apparatus was reducedto approximately 10⁻⁴ Pa. After that, over the first electrode 101,4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II) represented by Structural Formula (i) and molybdenum(VI) oxidewere deposited by co-evaporation by an evaporation method usingresistance heating, so that the hole-injection layer 111 was formed. Thethickness of the hole-injection layer 111 was set to 20 nm, and theweight ratio of DBT3P-II to molybdenum oxide was adjusted to 4:2. Notethat the co-evaporation method refers to an evaporation method in whichevaporation is carried out from a plurality of evaporation sources atthe same time in one treatment chamber.

Next, a film of 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP) which is represented by Structural Formula (ii)was formed to a thickness of 20 nm over the hole-injection layer 111 toform the hole-transport layer 112.

Further, the light-emitting layer 113 was formed over the hole-transportlayer 112 in the following manner2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II) represented by Structural Formula (vi),N-(1,1′-biphenyl-4-yl)-N-[4-(dibenzofuran-4-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: FrBBiF-II) represented by Structural Formula (100), andbis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(I II) (abbreviation: [Ir(tBuppm)₂(acac)]) represented byStructural Formula (vii) were deposited by co-evaporation to a thicknessof 20 nm such that the weight ratio of 2mDBTBPDBq-II to FrBBiF-II and[Ir(tBuppm)₂(acac)] was 0.7:0.3:0.05; then, 2mDBTBPDBq-II, FrBBiF-II,and [Ir(tBuppm)₂(acac)] were deposited by co-evaporation to a thicknessof 20 nm such that the weight ratio of 2mDBTBPDBq-II to FrBBiF-II and[Ir(tBuppm)₂(acac)] was 0.8:0.2:0.05.

Then, the electron-transport layer 114 was formed over thelight-emitting layer 113 in such a way that a 10 nm thick film of2mDBTBPDBq-II was formed and a 20 nm thick film of bathophenanthroline(abbreviation: BPhen) represented by Structural Formula (iv) was formed.

After the formation of the electron-transport layer 114, lithiumfluoride (LiF) was deposited by evaporation to a thickness of 1 nm, sothat the electron-injection layer 115 was formed. Lastly, aluminum wasdeposited by evaporation to a thickness of 200 nm to form the secondelectrode 102 functioning as a cathode. Thus, the light-emitting element2 in this example was fabricated.

Note that in all the above evaporation steps, evaporation was performedby a resistance-heating method.

The light-emitting element 2 was sealed using a glass substrate in aglove box containing a nitrogen atmosphere so as not to be exposed tothe air (specifically, a sealing material was applied onto an outer edgeof the element and heat treatment was performed at 80° C. for 1 hour atthe time of sealing). Then, initial characteristics of thelight-emitting element were measured. Note that the measurement wascarried out at room temperature (in an atmosphere kept at 25° C.).

FIG. 22 shows current density-luminance characteristics of thelight-emitting element 2; FIG. 23 shows luminance-current efficiencycharacteristics thereof; FIG. 24 shows voltage-luminance characteristicsthereof; FIG. 25 shows luminance-power efficiency characteristicsthereof; FIG. 26 shows luminance-external quantum efficiencycharacteristics thereof; and FIG. 27 shows an emission spectrum thereof.

Table 2 shows values of major characteristics of the light-emittingelement 2 at approximately 1000 cd/m².

TABLE 2 External Current Current Power Quantum Voltage Current DensityEfficiency Efficiency Efficiency (V) (mA) (mA/cm²) Chromaticity xChromaticity y (cd/A) (lm/W) (%) Light-emitting 2.8 0.04 0.9 0.42 0.57105.2 118.0 27.9 Element 2

The above results show that the light-emitting element 2 usingFrBBiF-II, which is the organic compound described in Embodiment 1, hasextremely favorable characteristics also when a light-emitting substanceemitting yellowish green phosphorescence is used as an emission centersubstance.

In particular, the external quantum efficiency was excellent; the valuewas kept high even in a high-luminance region. Besides, the drivevoltage was low, and as a result, an extremely high power efficiency of120 lm/W or more was achieved.

A reliability test was carried out, and the results thereof are shown inFIG. 28. In the reliability test, the light-emitting element 2 wasdriven under the conditions where the initial luminance was set to 5000cd/m² and the current density was constant. FIG. 28 shows a change innormalized luminance where the initial luminance is 100%. The resultsshow that a decrease in luminance over driving time of thelight-emitting element 2 is small, and thus the light-emitting element 2has favorable reliability.

Example 4

In this example, a light-emitting element (a light-emitting element 3)in whichN-(1,1′-biphenyl-4-yl)-N-[4-(dibenzofuran-4-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: FrBBiF-II) is used in a hole-transport layer will bedescribed. Chemical formulae of substances used in this example areshown below.

(Method for Fabricating Light-Emitting Element 3)

First, a film of indium tin oxide containing silicon oxide (ITSO) wasfruited over a glass substrate by a sputtering method, so that the firstelectrode 101 was formed. The thickness thereof was 110 nm and theelectrode area was 2 mm×2 mm. Here, the first electrode 101 is anelectrode that functions as an anode of the light-emitting element.

Next, as pretreatment for forming the light-emitting element over thesubstrate, UV ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for 1 hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for approximately 30 minutes.

Then, the substrate over which the first electrode 101 was formed wasfixed to a substrate holder provided in the vacuum evaporation apparatusso that the surface on which the first electrode 101 was formed faceddownward. The pressure in the vacuum evaporation apparatus was reducedto approximately 10⁻⁴ Pa. After that, over the first electrode 101,4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II) represented by Structural Formula (i) and molybdenum(VI) oxidewere deposited by co-evaporation by an evaporation method usingresistance heating, so that the hole-injection layer 111 was formed. Thethickness of the hole-injection layer 111 was set to 20 nm, and theweight ratio of DBT3P-II to molybdenum oxide was adjusted to 4:2. Notethat the co-evaporation method refers to an evaporation method in whichevaporation is carried out from a plurality of evaporation sources atthe same time in one treatment chamber.

Next, a film ofN-(1,1′-biphenyl-4-yl)-N-[4-(dibenzofuran-4-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: FrBBiF-II) represented by Structural Formula (100) wasformed to a thickness of 20 nm over the hole-injection layer 111 to formthe hole-transport layer 112.

Further, the light-emitting layer 113 was formed over the hole-transportlayer 112 in the following manner2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II) represented by Structural Formula (vi),N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine (abbreviation: PCBBiF) represented by Structural Formula(viii), and (acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)(abbreviation: [Ir(dppm)₂(acac)]) represented by Structural Formula (ix)were deposited by co-evaporation to a thickness of 20 nm such that theweight ratio of 2mDBTBPDBq-II to PCBBiF and [Ir(dppm)₂(acac)] was0.7:0.3:0.05; then, 2mDBTBPDBq-II, PCBBiF, and [Ir(dppm)₂(acac)] weredeposited by co-evaporation to a thickness of 20 nm such that the weightratio of 2mDBTBPDBq-II to PCBBiF and [Ir(dppm)₂(acac)] was 0.8:0.2:0.05.

Then, the electron-transport layer 114 was formed over thelight-emitting layer 113 in such a way that a 20 nm thick film of2mDBTBPDBq-II was formed and a 10 nm thick film of bathophenanthroline(abbreviation: BPhen) represented by Structural Formula (iv) was formed.

After the formation of the electron-transport layer 114, lithiumfluoride (LiF) was deposited by evaporation to a thickness of 1 nm, sothat the electron-injection layer 115 was formed. Lastly, aluminum wasdeposited by evaporation to a thickness of 200 nm to form the secondelectrode 102 functioning as a cathode. Thus, the light-emitting element3 in this example was fabricated.

Note that in all the above evaporation steps, evaporation was performedby a resistance-heating method.

The light-emitting element 3 was sealed using a glass substrate in aglove box containing a nitrogen atmosphere so as not to be exposed tothe air (specifically, a sealing material was applied onto an outer edgeof the element and heat treatment was performed at 80° C. for 1 hour atthe time of sealing). Then, initial characteristics of thelight-emitting element were measured. Note that the measurement wascarried out at room temperature (in an atmosphere kept at 25° C.).

FIG. 29 shows luminance-current efficiency characteristics of thelight-emitting element 3; FIG. 30 shows voltage-luminancecharacteristics thereof; FIG. 31 shows voltage-current characteristicsthereof; and FIG. 32 shows an emission spectrum thereof.

Table 3 shows values of major characteristics of the light-emittingelement 3 at approximately 1000 cd/m².

TABLE 3 External Current Current Power Quantum Voltage Current DensityEfficiency Efficiency Efficiency (V) (mA) (mA/cm²) Chromaticity xChromaticity y (cd/A) (lm/W) (%) Light-emitting 2.9 0.06 1.4 0.56 0.4481 88 30 Element 3

The above results show that the light-emitting element 3 in which thehole-transport layer includes FrBBiF-II, which is the organic compounddescribed in Embodiment 1, has extremely favorable characteristics whena light-emitting substance emitting orange phosphorescence is used as anemission center substance.

In particular, the external quantum efficiency was excellent; the valuewas kept high even in a high-luminance region.

A reliability test was carried out, and the results thereof are shown inFIG. 33. In the reliability test, the light-emitting element 3 wasdriven under the conditions where the initial luminance was set to 5000cd/m² and the current density was constant. FIG. 33 shows a change innormalized luminance where the initial luminance is 100%. The resultsshow that a decrease in luminance over driving time of thelight-emitting element 3 is small, and thus the light-emitting element 3has favorable reliability.

Example 5

In this example, a method for synthesizingN-(1,1′-biphenyl-4-yl)-N-[4-(dibenzothiophen-4-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: ThBBiF) represented by Structural Formula (200) will bedescribed.

First, 2.2 g (6.4 mmol) of 4-(4-bromophenyl)dibenzothiophene, 2.5 g (6.8mmol) of N-(1,1′-biphenyl-4-yl)-9,9-dimethyl-9H-fluoren-2-amine, and 1.9g (19.4 mmol) of sodium tert-butoxide were put in a 200-mL three-neckflask and the air in the flask was replaced with nitrogen. To thismixture were added 33.0 mL of toluene and 0.4 mL of a 10% hexanesolution of tri(tert-butyl)phosphine, the temperature was set to 60° C.,and 37 mg (0.1 mmol) of bis(dibenzylideneacetone)palladium(0) was added;then, the temperature was raised to 80° C. and stirring was performedfor 2.0 hours. After the stirring, suction filtration through Florisil,Celite, and alumina was carried out to give a filtrate. The filtrate wasconcentrated to give a solid. The solid was purified by silica gelcolumn chromatography (the developing solvent was hexane and toluene ina ratio of 3:1) to give a solid. The solid was recrystallized fromtoluene and hexane, so that 3.4 g of an objective solid was obtained ina yield of 85%.

Using a train sublimation method, 1.5 g of the obtained solid waspurified by sublimation. In the purification by sublimation, under apressure of 2.8 Pa and with an argon gas flow rate of 5.0 mL/min, thesolid was heated at 256° C. for 16.0 hours and then at 265° C. for 2.0hours. After the purification by sublimation, 1.4 g of a solid which wasthe object of the synthesis was obtained at a collection rate of 95%.

Results of measurement of the obtained solid by nuclear magneticresonance (¹H NMR) are shown below.

¹H NMR (CDCl₃, 500 MHz): δ=1.47 (s, 6H), 7.18 (dd, J=8.3 Hz, 2.5 Hz,1H), 7.27-7.35 (m, 8H), 7.41-7.50 (m, 5H), 7.51-7.57 (m, 4H), 7.61-7.68(m, 6H), 7.84-7.88 (m, 1H), 8.14 (dd, J=7.5 Hz, 1.0 Hz, 1H), 8.17-8.21(m, 1H).

FIGS. 34A and 34B are NMR charts. Note that FIG. 34B shows an enlargedpart of FIG. 34A in the range of 7.00 ppm to 8.25 ppm. The measurementresults confirmed thatN-(1,1′-biphenyl-4-yl)-N-[4-(dibenzothiophen-4-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: ThBBiF), which was the target substance, was obtained.

Thermogravimetry-differential thermal analysis (TG-DTA) of the obtainedThBBiF was performed. The measurement was conducted by using a highvacuum differential type differential thermal balance (TG/DTA 2410SA,manufactured by Bruker AXS K.K.). The measurement was carried out undera nitrogen stream (a flow rate of 200 mL/min) and a normal pressure at atemperature rising rate of 10° C./min. The relationship between weightand temperature (thermogravimety) shows that the 5% weight losstemperature is 403° C., which is indicative of high heat resistance.

Furthermore, ThBBiF was analyzed by liquid chromatography massspectrometry (LC/MS).

The analysis by LC/MS was carried out with Acquity UPLC (produced byWaters Corporation) and Xevo G2 T of MS (produced by WatersCorporation).

In the MS analysis, ionization was carried out by an electrosprayionization (ESI) method. Capillary voltage and sample cone voltage wereset to 3.0 kV and 30 V, respectively. Detection was performed in apositive mode.

A component which underwent the ionization under the above-mentionedconditions was collided with an argon gas in a collision cell todissociate into product ions. Energy (collision energy) for thecollision with argon was 50 eV. A mass range for the measurement wasm/z=100 to 1200. FIG. 35 shows the results.

<<Physical Properties of ThBBiF>>

FIG. 36A shows an absorption spectrum and an emission spectrum of ThBBiFin a toluene solution of ThBBiF, and FIG. 36B shows an absorptionspectrum and an emission spectrum of a thin film of ThBBiF. The spectrawere measured with a UV-visible spectrophotometer (V550, produced byJASCO Corporation). The spectra of ThBBiF in the toluene solution ofThBBiF were measured with a toluene solution of ThBBiF put in a quartzcell. The spectra of the thin film were measured with a sample preparedby deposition of ThBBiF on a quartz substrate by evaporation. Note thatin the case of the absorption spectrum of ThBBiF in the toluene solutionof ThBBiF, the absorption spectrum obtained by subtraction of theabsorption spectra of the quartz cell and toluene from the measuredspectra is shown in the drawing and that in the case of the absorptionspectrum of the thin film of ThBBiF, the absorption spectrum obtained bysubtraction of the absorption spectrum of the quartz substrate from themeasured spectra is shown in the drawing.

As shown in FIG. 36A, in the case of ThBBiF in the toluene solution, anabsorption peak was observed at approximately 358 nm, and an emissionwavelength peak was observed at approximately 411 nm (excitationwavelength: 362 nm). As shown in FIG. 36B, in the case of the thin filmof ThBBiF, absorption peaks were observed at approximately 362 nm, 289nm, 242 nm, and 210 nm, and an emission wavelength peak was observed atapproximately 431 nm (excitation wavelength: 363 nm). Thus, it was foundthat absorption and light emission of ThBBiF occur in extremely shortwavelength regions.

The ionization potential of ThBBiF in a thin film state was measured byphotoelectron spectroscopy (the measuring instrument: AC-2, manufacturedby Riken Keiki, Co., Ltd.) in the air. The obtained value of theionization potential was converted into a negative value, so that theHOMO level of ThBBiF was determined to be −5.60 eV. From the data of theabsorption spectrum of the thin film in FIG. 36B, the absorption edge ofThBBiF, which was obtained from Tauc plot with an assumption of directtransition, was 3.15 eV. Therefore, the optical energy gap of ThBBiF ina solid state was estimated at 3.15 eV; from the values of the HOMOlevel obtained above and this energy gap, the LUMO level of ThBBiF wasestimated at −2.45 eV. This reveals that ThBBiF in the solid state hasan energy gap as wide as 3.15 eV.

Example 6

In this example, the light-emitting element (a light-emitting element 4)described in Embodiment 3 will be described. Note that in thelight-emitting element 4,N-(1,1′-biphenyl-4-yl)-N-[4-(dibenzothiophen-4-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: ThBBiF), which is the organic compound described inEmbodiment 1, was used as the second organic compound in thelight-emitting layer 113,2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II) was used as the first organic compound,andbis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(I II) (abbreviation: [Ir(tBuppm)₂(acac)]) was used as anemission center substance emitting phosphorescence. Chemical formulae ofmaterials used in this example are shown below.

(Method for Fabricating Light-Emitting Element 4)

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate by a sputtering method, so that the firstelectrode 101 was formed. The thickness thereof was 110 nm and theelectrode area was 2 mm×2 mm. Here, the first electrode 101 is anelectrode that functions as an anode of the light-emitting element.

Next, as pretreatment for forming the light-emitting element over thesubstrate, UV ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for 1 hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for approximately 30 minutes.

Then, the substrate over which the first electrode 101 was formed wasfixed to a substrate holder provided in the vacuum evaporation apparatusso that the surface on which the first electrode 101 was formed faceddownward. The pressure in the vacuum evaporation apparatus was reducedto approximately 10⁻⁴ Pa. After that, over the first electrode 101,4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II) represented by Structural Formula (1) and molybdenum(VI) oxidewere deposited by co-evaporation by an evaporation method usingresistance heating, so that the hole-injection layer 111 was formed. Thethickness of the hole-injection layer 111 was set to 20 nm, and theweight ratio of DBT3P-II to molybdenum oxide was adjusted to 4:2.

Next, a film of 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP) which is represented by Structural Formula (ii)was formed to a thickness of 20 nm over the hole-injection layer 111 toform the hole-transport layer 112.

Further, the light-emitting layer 113 was formed over the hole-transportlayer 112 in the following manner2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II) represented by Structural Formula (vi),N-(1,1′-biphenyl-4-yl)-N-[4-(dibenzothiophen-4-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: ThBBiF) represented by Structural Formula (200), andbis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(I II) (abbreviation: [Ir(tBuppm)₂(acac)]) represented byStructural Formula (vii) were deposited by co-evaporation to a thicknessof 20 nm such that the weight ratio of 2mDBTBPDBq-II to ThBBiF and[Ir(tBuppm)₂(acac)] was 0.7:0.3:0.05; then, 2mDBTBPDBq-II, ThBBiF, and[Ir(tBuppm)₂(acac)] were deposited by co-evaporation to a thickness of20 nm such that the weight ratio of 2mDBTBPDBq-II to ThBBiF and[Ir(tBuppm)₂(acac)] was 0.8:0.2:0.05.

Then, the electron-transport layer 114 was formed over thelight-emitting layer 113 in such a way that a 20 nm thick film of2mDBTBPDBq-II was formed and a 10 nm thick film of bathophenanthroline(abbreviation: BPhen) represented by Structural Formula (iv) was formed.

After the formation of the electron-transport layer 114, lithiumfluoride (LiF) was deposited by evaporation to a thickness of 1 nm, sothat the electron-injection layer 115 was formed. Lastly, aluminum wasdeposited by evaporation to a thickness of 200 nm to form the secondelectrode 102 functioning as a cathode. Thus, the light-emitting element4 in this example was fabricated.

Note that in all the above evaporation steps, evaporation was performedby a resistance-heating method.

The light-emitting element 4 was sealed using a glass substrate in aglove box containing a nitrogen atmosphere so as not to be exposed tothe air (specifically, a sealing material was applied onto an outer edgeof the element and heat treatment was performed at 80° C. for 1 hour atthe time of sealing). Then, initial characteristics of thelight-emitting element were measured. Note that the measurement wascarried out at room temperature (in an atmosphere kept at 25° C.).

FIG. 37 shows current density-luminance characteristics of thelight-emitting element 4; FIG. 38 shows luminance-current efficiencycharacteristics thereof; FIG. 39 shows voltage-luminance characteristicsthereof; FIG. 40 shows luminance-power efficiency characteristicsthereof; FIG. 41 shows luminance-external quantum efficiencycharacteristics thereof; and FIG. 42 shows an emission spectrum thereof.

Table 4 shows values of major characteristics of the light-emittingelement 4 at approximately 1000 cd/m².

TABLE 4 External Current Current Power Quantum Voltage Current DensityEfficiency Efficiency Efficiency (V) (mA) (mA/cm²) Chromaticity xChromaticity y (cd/A) (lm/W) (%) Light-emitting 3.1 0.04 1.1 0.42 0.56103 104 28 Element 4

The above results show that the light-emitting element 4 using ThBBiF,which is the organic compound described in Embodiment 1, has extremelyfavorable characteristics also when a light-emitting substance emittingyellowish green phosphorescence is used as an emission center substance.

In particular, the external quantum efficiency was excellent; the valuewas kept high even in a high-luminance region. Besides, the drivevoltage was low, and as a result, an extremely high power efficiency of1201m/W or more was achieved.

A reliability test was carried out, and the results thereof are shown inFIG. 43. In the reliability test, the light-emitting element 4 wasdriven under the conditions where the initial luminance was set to 5000cd/m² and the current density was constant. FIG. 43 shows a change innormalized luminance where the initial luminance is 100%. The resultsshow that a decrease in luminance over driving time of thelight-emitting element 4 is small, and thus the light-emitting element 4has favorable reliability.

Example 7

In this example, the light-emitting element (a light-emitting element 5)described in Embodiment 2 will be described. Note that in thelight-emitting element 5,N-(1,1′-biphenyl-4-yl)-N-[4-(dibenzothiophen-4-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: ThBBiF), which is the organic compound described inEmbodiment 1, was used as the second organic compound in thelight-emitting layer 113, and4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation: 4,6mCzP2Pm)was used as the first organic compound. Chemical formulae of materialsused in this example are shown below.

(Method for Fabricating Light-Emitting Element 5)

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate by a sputtering method, so that the firstelectrode 101 was formed. The thickness thereof was 110 nm and theelectrode area was 2 mm×2 mm. Here, the first electrode 101 is anelectrode that functions as an anode of the light-emitting element.

Next, as pretreatment for forming the light-emitting element over thesubstrate, UV ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for 1 hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for approximately 30 minutes.

Then, the substrate over which the first electrode 101 was formed wasfixed to a substrate holder provided in the vacuum evaporation apparatusso that the surface on which the first electrode 101 was formed faceddownward. The pressure in the vacuum evaporation apparatus was reducedto approximately le Pa. After that, over the first electrode 101,4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II) represented by Structural Formula (1) and molybdenum(VI) oxidewere deposited by co-evaporation by an evaporation method usingresistance heating, so that the hole-injection layer 111 was formed. Thethickness of the hole-injection layer 111 was set to 20 nm, and theweight ratio of DBT3P-II to molybdenum oxide was adjusted to 4:2. Notethat the co-evaporation method refers to an evaporation method in whichevaporation is carried out from a plurality of evaporation sources atthe same time in one treatment chamber.

Next, a film of 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP) which is represented by Structural Formula (ii)was formed to a thickness of 20 nm over the hole-injection layer 111 toform the hole-transport layer 112.

Further, over the hole-transport layer 112,4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation: 4,6mCzP2Pm)represented by Structural Formula (iii) andN-(1,1′-biphenyl-4-yl)-N-[4-(dibenzothiophen-4-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: ThBBiF) represented by Structural Formula (200) weredeposited by co-evaporation to a thickness of 40 nm such that the weightratio of 4,6mCzP2Pm to ThBBiF was 0.8:0.2, whereby the light-emittinglayer 113 was formed.

Then, the electron-transport layer 114 was formed over thelight-emitting layer 113 in such a way that a 20 nm thick film of4,6mCzP2Pm was formed and a 10 nm thick film of bathophenanthroline(abbreviation: BPhen) represented by Structural Formula (iv) was formed.

After the formation of the electron-transport layer 114, lithiumfluoride (LiF) was deposited by evaporation to a thickness of 1 nm, sothat the electron-injection layer 115 was formed. Lastly, aluminum wasdeposited by evaporation to a thickness of 200 nm to form the secondelectrode 102 functioning as a cathode. Thus, the light-emitting element5 in this example was fabricated.

Note that in all the above evaporation steps, evaporation was performedby a resistance-heating method.

The light-emitting element 5 was sealed using a glass substrate in aglove box containing a nitrogen atmosphere so as not to be exposed tothe air (specifically, a sealing material was applied onto an outer edgeof the element and heat treatment was performed at 80° C. for 1 hour atthe time of sealing). Then, initial characteristics of thelight-emitting element were measured. Note that the measurement wascarried out at room temperature (in an atmosphere kept at 25° C.).

FIG. 44 shows current density-luminance characteristics of thelight-emitting element 5; FIG. 45 shows luminance-current efficiencycharacteristics thereof; FIG. 46 shows voltage-luminance characteristicsthereof; FIG. 47 shows luminance-power efficiency characteristicsthereof; FIG. 48 shows luminance-external quantum efficiencycharacteristics thereof; and FIG. 49 shows emission spectra thereof.

Table 5 shows values of major characteristics of the light-emittingelement 5 at approximately 1000 cd/m².

TABLE 5 External Current Current Power Quantum Voltage Current DensityEfficiency Efficiency Efficiency (V) (mA) (mA/cm²) Chromaticity xChromaticity y (cd/A) (lm/W) (%) Light-emitting 4.6 0.97 24.2 0.25 0.464.5 3.1 1.5 Element 5

The above results show that the light-emitting element 5 using ThBBiF,which is the organic compound described in Embodiment 1, has favorablecharacteristics. Specifically, according to FIG. 48 showing the externalquantum efficiency, the light-emitting element 5 has an efficiency farexceeding 5% in a low-luminance region. It is said that the extractionefficiency of a light-emitting element like the light-emitting elementin this example which is not designed to enhance extraction efficiencyis approximately 20% to 30%. The theoretical limit of the internalquantum efficiency of fluorescence, which is based on the generationratio of singlet excitons generated by current excitation, is 25%. Thus,the theoretical limit of the external quantum efficiency of afluorescent light-emitting element is calculated to be 5% to 7.5%. Itcan be found that the external quantum efficiency of the light-emittingelement 5 in a low-luminance region exceeds the theoretical limit.

The above results suggest that an exciplex formed by the first organiccompound and the second organic compound (4,6mCzP2Pm and ThBBiF) emitslight with high efficiency and that the light includes delayedfluorescence components. The light-emitting element 5 was able to emitlight with high emission efficiency owing to delayed fluorescence thatoccurred efficiently via reverse intersystem crossing from a tripletexcited state to a singlet excited state.

REFERENCE NUMERALS

101: first electrode, 102: second electrode, 103: layer containing anorganic compound, 111: hole-injection layer, 112: hole-transport layer,113: light-emitting layer, 114: electron-transport layer, 115:electron-injection layer, 400: substrate, 401: first electrode, 403:layer containing an organic compound, 404: second electrode, 405:sealing material, 407: sealing substrate, 412: pad, 420: IC chip, 501:first electrode, 502: second electrode, 511: first light-emitting unit,512: second light-emitting unit, 513: charge-generation layer, 601:driver circuit portion (source line driver circuit), 602: pixel portion,603: driver circuit portion (gate line driver circuit), 604: sealingsubstrate, 605: sealing material, 607: space, 608: wiring, 609: FPC(flexible printed circuit), 610: element substrate, 611: switching TFT,612: current controlling TFT, 613: first electrode, 614: insulator, 616:layer containing an organic compound, 617: second electrode, 618:light-emitting element, 623: n-channel TFT, 624: p-channel TFT, 625:desiccant, 901: housing, 902: liquid crystal layer, 903: backlight unit,904: housing, 905: driver IC, 906: terminal, 951: substrate, 952:electrode, 953: insulating layer, 954: partition layer, 955: layercontaining an organic compound, 956: electrode, 1001: substrate, 1002:base insulating film, 1003: gate insulating film, 1006: gate electrode,1007: gate electrode, 1008: gate electrode, 1020: first interlayerinsulating film, 1021: second interlayer insulating film, 1022:electrode, 1024W: first electrode of a light-emitting element, 1024R:first electrode of a light-emitting element, 1024G: first electrode of alight-emitting element, 1024B: first electrode of a light-emittingelement, 1025: partition, 1028: layer containing an organic compound,1029: second electrode of a light-emitting element, 1031: sealingsubstrate, 1032: sealing material, 1033: transparent base material,1034R: red coloring layer, 1034G: green coloring layer, 1034B: bluecoloring layer, 1035: black layer (black matrix), 1036: overcoat layer,1037: third interlayer insulating film, 1040: pixel portion, 1041:driver circuit portion, 1042: peripheral portion, 2001: housing, 2002:light source, 3001: lighting device, 5000: display region, 5001: displayregion, 5002: display region, 5003: display region, 5004: displayregion, 5005: display region, 7101: housing, 7103: display portion,7105: stand, 7107: display portion, 7109: operation key, 7110: remotecontroller, 7201: main body, 7202: housing, 7203: display portion, 7204:keyboard, 7205: external connection port, 7206: pointing device, 7210:second display portion, 7301: housing, 7302: housing, 7303: jointportion, 7304: display portion, 7305: display portion, 7306: speakerportion, 7307: recording medium insertion portion, 7308: LED lamp, 7309:operation key, 7310: connection terminal, 7311: sensor, 7401: housing,7402: display portion, 7403: operation button, 7404: external connectionport, 7405: speaker, 7406: microphone, 9033: clasp, 9034: switch, 9035:power switch, 9036: power saver switch, 9038: operation switch, 9630:housing, 9631: display portion, 9631 a: display portion, 9631 b: displayportion, 9632 a: touchscreen region, 9632 b: touchscreen region, 9633:solar cell, 9634: charge and discharge control circuit, 9635: battery,9636: DC-to-DC converter, 9637: operation key, 9638: converter, and9639: button.

This application is based on Japanese Patent Application serial no.2013-064278 filed with Japan Patent Office on Mar. 26, 2013, the entirecontents of which are hereby incorporated by reference.

1. An organic compound represented by a formula (G1):

wherein: A represents one of a dibenzofuranyl group and adibenzothiophenyl group; and R¹ and R² each separately represent one ofhydrogen, an alkyl group having 1 to 6 carbon atoms, and a phenyl group.2. The organic compound according to claim 1, wherein the organiccompound is represented by a formula (G2):


3. The organic compound according to claim 1, wherein when both R¹ andR² are phenyl groups, the phenyl groups are bonded to each other to forma spirofluorene skeleton.
 4. The organic compound according to claim 1,wherein A represents one of groups represented by formulae (A-1) to(A-4):


5. The organic compound according to claim 1, wherein A represents agroup represented by a formula (A-1) or (A-2):


6. The organic compound according to claim 1, wherein A represents agroup represented by a formula (A-1):


7. The organic compound according to claim 1, wherein R¹ and R² eachseparately represent one of groups represented by formulae (R-1) to(R-12):


8. The organic compound according to claim 1, wherein both R¹ and R² aremethyl groups.
 9. The organic compound according to claim 1, wherein theorganic compound is represented by a formula (100):


10. A light-emitting element comprising the organic compound accordingto claim 1, wherein: the light-emitting element comprises: a pair ofelectrodes; and a layer between the pair of electrodes; and the layercontains the organic compound.
 11. A light-emitting element comprisingthe organic compound according to claim 1, wherein: the light-emittingelement comprises: a pair of electrodes; and a layer between the pair ofelectrodes; the layer comprises a light-emitting layer; and thelight-emitting layer contains the organic compound.
 12. A light-emittingelement comprising the organic compound according to claim 1, wherein:the light-emitting element comprises: a pair of electrodes; and a layerbetween the pair of electrodes; the layer comprises a light-emittinglayer; the light-emitting layer contains a first organic compound and asecond organic compound; the first organic compound has anelectron-transport property; and the second organic compound is theorganic compound.
 13. A light-emitting element comprising the organiccompound according to claim 1, wherein: the light-emitting elementcomprises: a pair of electrodes; and a layer between the pair ofelectrodes; the layer comprises a light-emitting layer; thelight-emitting layer contains a first organic compound, a second organiccompound, and a phosphorescent substance; the first organic compound hasan electron-transport property; and the second organic compound is theorganic compound.
 14. The light-emitting element according to claim 13,wherein the first organic compound and the second organic compound forman exciplex.
 15. The light-emitting element according to claim 14,wherein triplet excitation energy of each of the first organic compoundand the second organic compound is higher than light energy of lightemitted by the exciplex formed by the first organic compound and thesecond organic compound.
 16. A display module comprising thelight-emitting element according to claim
 10. 17. A lighting modulecomprising the light-emitting element according to claim
 10. 18. Alight-emitting device comprising the light-emitting element according toclaim 10, wherein the light-emitting device comprises a unit configuredto control the light-emitting element.
 19. A display device comprisingthe light-emitting element according to claim 10, wherein: the displaydevice comprises: a display portion including the light-emittingelement; and a unit configured to control the light-emitting element.20. A lighting device comprising the light-emitting element according toclaim 10, wherein: the lighting device comprises: a lighting portionincluding the light-emitting element; and a unit configured to controlthe light-emitting element.
 21. An electronic device comprising thelight-emitting element according to claim 10.